How to reproduce:
1. Create a prefab
2. Delete the prefab in Assets folder (scene object turns red)
3. Break prefab instance for the object

Actual result: Deleting a prefab referenced in the scene and then breaking the prefab instance for the referencing object shows the Prefab Buttons "Select" and "Revert" in the inspector, though there's no prefab anymore. Relesecting the object removes the buttons and shows the regular Inspector view.

Reproduced with: 2017.2.1p2, 2017.3.1f1, 2018.1.0b4