So did you use the oven? Did you take the shit out? How dry was it? After everyone left the room, did you lick it? How did you clean the oven? If you used the oven's auto-clean, did it stink up the kitchen?
As for finding the poop-etrator, I'd suspect any heterosexual male friends that you may have. A heterosexual man doesn't understand the deep sensual feelings that you people have for shit, and probably thought he was being funny. "Ha, ha! It is poop! And it is in his oven! I am a genius!" OTOH, if any of your queer buddies are shit pigs, they may been intended to heat-up the shit to eat, and merely forgotten about it. You can't ignore any possibilities!
It may have even been one of the fags who "found" it...
...when fits of creativity run strong, more than one programmer or writer has been known to abandon the desktop for the more spacious floor. - Fred Brooks, Jr.