Why is unity deleted my assets?

Hey all, thank you in advance for any help. I am using Unity 3.5.2 as it's the version my university uses. The other day I was doing an animation and alt tabbed in and out of unity. When I alt tabbed in it had deleted the asset I was animating from the scene and the prefab had a grey box in the project window which was empty.

It happened twice and I got fed up and went home, assuming it was just a freak accident. But the same thing just happened on my computer and my install of Unity. Same thing, made a change to a prefab in the scene, alt tabbed to script reference, alt tabbed back and both prefab and in scene asset were gone.

Three times in a week is not a freak accident anymore. Luckily I have not lost much work as a result but I'm about to start a major project for university that will span months and need to figure this out. I couldn't find anyone else having the same issue online so I was hoping someone knew something I didn't.

Cheers guys. x