diff --git a/auth/iam.go b/auth/iam.go
index 210308ae..8dba2550 100644
--- a/auth/iam.go
+++ b/auth/iam.go
@@ -30,7 +30,7 @@ func resolveAccountsByLookup(accessKeyIDs []string, getUserAccount func(string)
 	for _, access := range accessKeyIDs {
 		_, err := getUserAccount(access)
 		if err != nil {
-			if err == ErrNoSuchUser || err == s3err.GetAPIError(s3err.ErrAdminUserNotFound) {
+			if err == ErrNoSuchUser {
 				missing = append(missing, access)
 				continue
 			}
diff --git a/auth/iam_ipa.go b/auth/iam_ipa.go
index a490077a..ba3569a8 100644
--- a/auth/iam_ipa.go
+++ b/auth/iam_ipa.go
@@ -137,6 +137,9 @@ func (ipa *IpaIAMService) GetUserAccount(access string) (Account, error) {
 	}{}
 
 	err = ipa.rpc(req, &userResult)
+	if errors.Is(err, errRpcNotFound) {
+		return Account{}, ErrNoSuchUser
+	}
 	if err != nil {
 		return Account{}, err
 	}
@@ -176,12 +179,16 @@ func (ipa *IpaIAMService) GetUserAccount(access string) (Account, error) {
 	// ipaclient/plugins/vault.py's _do_internal - so try the default first
 	// and fall back to OAEP so both KRA configurations keep working.
 	data, err := ipa.retrieveVaultSecret(access, session_key, false)
-	if err != nil {
+	if err != nil && !errors.Is(err, errRpcNotFound) {
 		debuglogger.IAMLogf("ipa vault_retrieve_internal with PKCS1v15 session key wrap failed, retrying with OAEP: %v", err)
 		data, err = ipa.retrieveVaultSecret(access, session_key, true)
-		if err != nil {
-			return account, err
-		}
+	}
+	if errors.Is(err, errRpcNotFound) {
+		// a user without the vault has no secret, so it isn't an S3 account
+		return Account{}, ErrNoSuchUser
+	}
+	if err != nil {
+		return account, err
 	}
 
 	aes, err := aes.NewCipher(session_key)
@@ -332,6 +339,12 @@ func (p rpcResponse) String() string {
 
 var errRpc = errors.New("IPA RPC error")
 
+// errRpcNotFound is the IPA RPC error for an object that does not exist,
+// such as a user or a user's vault (FreeIPA's NotFound error)
+var errRpcNotFound = fmt.Errorf("not found: %w", errRpc)
+
+const ipaErrNotFoundCode = 4001
+
 func (ipa *IpaIAMService) rpc(req rpcRequest, value any) error {
 	err := ipa.login()
 	if err != nil {
@@ -403,6 +416,12 @@ func (ipa *IpaIAMService) rpcInternal(req rpcRequest) (rpcResponse, error) {
 		return rpcResponse{}, err
 	}
 	if string(result.Error) != "null" {
+		rpcErr := struct {
+			Code int `json:"code"`
+		}{}
+		if json.Unmarshal(result.Error, &rpcErr) == nil && rpcErr.Code == ipaErrNotFoundCode {
+			return rpcResponse{}, fmt.Errorf("%s: %w", string(result.Error), errRpcNotFound)
+		}
 		return rpcResponse{}, fmt.Errorf("%s: %w", string(result.Error), errRpc)
 	}
 
diff --git a/auth/iam_ipa_test.go b/auth/iam_ipa_test.go
new file mode 100644
index 00000000..f6520639
--- /dev/null
+++ b/auth/iam_ipa_test.go
@@ -0,0 +1,106 @@
+// Copyright 2026 Versity Software
+// This file is licensed under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package auth
+
+import (
+	"crypto/rand"
+	"crypto/rsa"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// FreeIPA JSON-RPC response bodies, as returned by a FreeIPA 4.13 server.
+const (
+	ipaUserShowNotFound = `{"result": null, "error": {"code": 4001, "message": "nosuchkeyid0000000: user not found", "data": {"reason": "nosuchkeyid0000000: user not found"}, "name": "NotFound"}, "id": 0, "principal": "admin@EXAMPLE.TEST", "version": "4.13.1"}`
+	ipaUserShowFound    = `{"result": {"result": {"uid": ["novaultuser"], "uidnumber": ["1828600004"], "gidnumber": ["1828600004"]}, "value": "novaultuser", "summary": null}, "error": null, "id": 0, "principal": "admin@EXAMPLE.TEST", "version": "4.13.1"}`
+	ipaVaultNotFound    = `{"result": null, "error": {"code": 4001, "message": "s3secret: vault not found", "data": {"reason": "s3secret: vault not found"}, "name": "NotFound"}, "id": 1, "principal": "admin@EXAMPLE.TEST", "version": "4.13.1"}`
+	ipaVersionError     = `{"result": null, "error": {"code": 901, "message": "9.999 client incompatible with 2.257 server at 'https://ipa.example.test/ipa/xml'", "data": {"cver": "9.999", "sver": "2.257", "server": "https://ipa.example.test/ipa/xml"}, "name": "VersionError"}, "id": 0, "principal": "admin@EXAMPLE.TEST", "version": "4.13.1"}`
+)
+
+func TestIpaIAMServiceGetUserAccountNotFound(t *testing.T) {
+	tests := []struct {
+		name       string
+		responses  map[string]string
+		noSuchUser bool
+	}{
+		{
+			name:       "unknown user",
+			responses:  map[string]string{"user_show/1": ipaUserShowNotFound},
+			noSuchUser: true,
+		},
+		{
+			name: "user without vault",
+			responses: map[string]string{
+				"user_show/1":               ipaUserShowFound,
+				"vault_retrieve_internal/1": ipaVaultNotFound,
+			},
+			noSuchUser: true,
+		},
+		{
+			name:      "other rpc error",
+			responses: map[string]string{"user_show/1": ipaVersionError},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ipa := newFakeIpaIAMService(t, tt.responses)
+
+			_, err := ipa.GetUserAccount("NOSUCHKEYID0000000")
+			if tt.noSuchUser {
+				assert.Equal(t, ErrNoSuchUser, err)
+			} else {
+				assert.ErrorIs(t, err, errRpc)
+				assert.NotErrorIs(t, err, ErrNoSuchUser)
+			}
+		})
+	}
+}
+
+// newFakeIpaIAMService returns an IpaIAMService talking to a fake FreeIPA
+// server that answers each JSON-RPC method with the given response body.
+func newFakeIpaIAMService(t *testing.T, responses map[string]string) *IpaIAMService {
+	t.Helper()
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == "/ipa/session/login_password" {
+			return
+		}
+		var req struct {
+			Method string `json:"method"`
+		}
+		assert.NoError(t, json.NewDecoder(r.Body).Decode(&req))
+		resp, ok := responses[req.Method]
+		assert.True(t, ok, "unexpected IPA method %q", req.Method)
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(resp))
+	}))
+	t.Cleanup(srv.Close)
+
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	assert.NoError(t, err)
+
+	return &IpaIAMService{
+		client:          *srv.Client(),
+		version:         IpaVersion,
+		host:            srv.URL,
+		vaultName:       "s3secret",
+		kraTransportKey: &key.PublicKey,
+		rootAcc:         Account{Access: "root"},
+	}
+}
diff --git a/auth/iam_single.go b/auth/iam_single.go
index 26e2c317..1e42103a 100644
--- a/auth/iam_single.go
+++ b/auth/iam_single.go
@@ -37,12 +37,12 @@ func (IAMServiceSingle) CreateAccount(account Account) error {
 }
 
 // GetUserAccount returns root account, if the root access key
-// is provided and "ErrAdminUserNotFound" otherwise
+// is provided and ErrNoSuchUser otherwise
 func (s IAMServiceSingle) GetUserAccount(access string) (Account, error) {
 	if access == s.root.Access {
 		return s.root, nil
 	}
-	return Account{}, s3err.GetAPIError(s3err.ErrAdminUserNotFound)
+	return Account{}, ErrNoSuchUser
 }
 
 // ResolveAccounts returns the subset of accessKeyIDs that do not exist.
diff --git a/auth/iam_single_test.go b/auth/iam_single_test.go
new file mode 100644
index 00000000..78a17e44
--- /dev/null
+++ b/auth/iam_single_test.go
@@ -0,0 +1,52 @@
+// Copyright 2026 Versity Software
+// This file is licensed under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package auth
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/versity/versitygw/internal/sigv4auth"
+)
+
+func TestIAMServiceSingleGetUserAccount(t *testing.T) {
+	root := Account{Access: "root", Secret: "rootsecret", Role: RoleAdmin}
+	iam := NewIAMServiceSingle(root)
+
+	acc, err := iam.GetUserAccount("root")
+	assert.NoError(t, err)
+	assert.Equal(t, root, acc)
+
+	_, err = iam.GetUserAccount("NOSUCHKEYID0000000")
+	assert.Equal(t, ErrNoSuchUser, err)
+}
+
+// The S3 auth middlewares render InvalidAccessKeyId only when
+// ResolveDerivedKey returns exactly ErrNoSuchUser.
+func TestIAMServiceSingleResolveDerivedKeyUnknownAccess(t *testing.T) {
+	root := Account{Access: "root", Secret: "rootsecret", Role: RoleAdmin}
+	iam := NewIAMServiceSingle(root)
+
+	_, _, err := ResolveDerivedKey(iam, root, "NOSUCHKEYID0000000", "", "20260928", "us-east-1", sigv4auth.ServiceS3)
+	assert.Equal(t, ErrNoSuchUser, err)
+}
+
+func TestIAMServiceSingleResolveAccounts(t *testing.T) {
+	iam := NewIAMServiceSingle(Account{Access: "root", Secret: "rootsecret", Role: RoleAdmin})
+
+	missing, err := iam.ResolveAccounts([]string{"root", "user1", "user2"})
+	assert.NoError(t, err)
+	assert.Equal(t, []string{"user1", "user2"}, missing)
+}
diff --git a/auth/iam_vault.go b/auth/iam_vault.go
index 23a2ae34..c41747f9 100644
--- a/auth/iam_vault.go
+++ b/auth/iam_vault.go
@@ -245,12 +245,18 @@ func (vt *VaultIAMService) GetUserAccount(access string) (Account, error) {
 	if err != nil {
 		reauthErr := vt.reAuthIfNeeded(err)
 		if reauthErr != nil {
+			if vault.IsErrorStatus(err, http.StatusNotFound) {
+				return Account{}, ErrNoSuchUser
+			}
 			return Account{}, reauthErr
 		}
 		// retry once after re-auth
 		resp, err = vt.client.Secrets.KvV2Read(context.Background(),
 			vt.secretStoragePath+"/"+access, vt.kvReqOpts...)
 		if err != nil {
+			if vault.IsErrorStatus(err, http.StatusNotFound) {
+				return Account{}, ErrNoSuchUser
+			}
 			return Account{}, err
 		}
 	}
diff --git a/auth/iam_vault_test.go b/auth/iam_vault_test.go
new file mode 100644
index 00000000..19929c48
--- /dev/null
+++ b/auth/iam_vault_test.go
@@ -0,0 +1,62 @@
+// Copyright 2026 Versity Software
+// This file is licensed under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package auth
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// Vault answers a KV v2 read of a missing secret with 404 and this body.
+func TestVaultIAMServiceGetUserAccountUnknownReturnsErrNoSuchUser(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/v1/kv-v2/data/users/NOSUCHKEYID0000000", r.URL.Path)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusNotFound)
+		w.Write([]byte(`{"errors":[]}`))
+	}))
+	defer srv.Close()
+
+	iam, err := NewVaultIAMService(Account{Access: "root"}, srv.URL, "", "users", "", "", "", "", "token", "", "", "", "", "")
+	assert.NoError(t, err)
+
+	_, err = iam.GetUserAccount("NOSUCHKEYID0000000")
+	assert.Equal(t, ErrNoSuchUser, err)
+
+	missing, err := iam.ResolveAccounts([]string{"NOSUCHKEYID0000000"})
+	assert.NoError(t, err)
+	assert.Equal(t, []string{"NOSUCHKEYID0000000"}, missing)
+}
+
+// Any Vault error other than 404, such as a token whose policy doesn't cover
+// the path, must not be mistaken for a missing account.
+func TestVaultIAMServiceGetUserAccountPermissionDenied(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusForbidden)
+		w.Write([]byte(`{"errors":["permission denied"]}`))
+	}))
+	defer srv.Close()
+
+	iam, err := NewVaultIAMService(Account{Access: "root"}, srv.URL, "", "users", "", "", "", "", "token", "", "", "", "", "")
+	assert.NoError(t, err)
+
+	_, err = iam.GetUserAccount("user1")
+	assert.Error(t, err)
+	assert.NotEqual(t, ErrNoSuchUser, err)
+}