diff --git a/tests/mock_vws/test_invalid_given_id.py b/tests/mock_vws/test_invalid_given_id.py
index 067867f75..9db6c8cac 100644
--- a/tests/mock_vws/test_invalid_given_id.py
+++ b/tests/mock_vws/test_invalid_given_id.py
@@ -34,6 +34,9 @@ def test_not_real_id(
         target ID
         of a target which does not exist.
         """
+        # This shared check only covers endpoints that end in target_id,
+        # such as /targets/{target_id}. Endpoints with trailing segments
+        # are covered by endpoint-specific tests.
         if not endpoint.path_url.endswith(target_id):
             return
 
diff --git a/tests/mock_vws/test_vumark_generation_api.py b/tests/mock_vws/test_vumark_generation_api.py
index e529bd479..8705713ce 100644
--- a/tests/mock_vws/test_vumark_generation_api.py
+++ b/tests/mock_vws/test_vumark_generation_api.py
@@ -195,6 +195,23 @@ def test_empty_instance_id(
             == ResultCodes.INVALID_INSTANCE_ID.value
         )
 
+    @staticmethod
+    def test_unknown_target(
+        vumark_vuforia_database: VuMarkCloudDatabase,
+    ) -> None:
+        """An unknown target_id returns UnknownTarget."""
+        response = _make_vumark_request(
+            server_access_key=vumark_vuforia_database.server_access_key,
+            server_secret_key=vumark_vuforia_database.server_secret_key,
+            target_id=uuid4().hex,
+            instance_id=uuid4().hex,
+            accept=VuMarkAccept.PNG,
+        )
+
+        assert response.status_code == HTTPStatus.NOT_FOUND
+        response_json = response.json()
+        assert response_json["result_code"] == ResultCodes.UNKNOWN_TARGET.value
+
     @staticmethod
     def test_non_vumark_database(
         vuforia_database: CloudDatabase,