How to reproduce:
1. Open attached project ("SpritePreviewBug.zip")
2. Create Empty GameObject
3. Add SpriteMask to that GameObject (set Sprite to "Left")
4. Add SpriteRenderer to that GameObject (set Sprite to any, it will be the one that shows up)
5. Change the position of that GameObject to 2,0,0 (the position must be not 0,0,0 or you will not see the bugged Sprite behind)
6. Turn that GameObject into a prefab
7. In Project window select newly created prefab. (it will appear in the scene, even though it should not)
8. Enter and leave Play mode
9. Now the sprite, which showed up in step 7, stays in the Scene/Game views (stays until the project is restarted)

Expected result: Sprite's preview is shown in Inspector's window
Actual result: when the prefab, in Project window, is selected its preview is shown in Scene/Game views

Reproducible with: 2017.1.3p3, 2017.2.2p2, 2017.3.2f1, 2017.4.1f1, 2018.1.0b13
Not reproducible with: 2018.2.0a7

Fixed in: 2018.2.0a1 Unity version