Well, yeah, it would have to be very, very dirty, possibly scratched too. I've seen that amount of dirt before, it's not that hard to get it if you don't care for the body and toss it on the junk pile with mirror exposed. But that can't be the reason since OP mentions the mirror in his first post and he doesn't mention anything suspicious there. I somehow missed or forgot that part of his post until now.

Here's another wild guess. The lens - OP didn't say if he had checked the lens, right? Wild mushrooms. Haze.

edit: Just missed OP's post above.