If you have a prefab open in Prefab Mode and drag an object from the Project View to the Hierarchy, it will get added to the hierarchy in the currently active scene.

1. Install Unity
2. Open the attached Project
3. Open TestScene.scene
4. Find "Prefab01" in the Project view and open in in Prefab Mode
3. Drag "TestSprite" from the Project view into the Hierarchy
4. Switch back from Prefab Mode to TestScene.scene

Result:
"TestSprite" is added to the scene.

Expected Result:
No objects should be added to the active scene if they are dragged into the Hierarchy while the user is in Prefab Mode.