Well, I found it, but I still have no idea what it once was. Most of the Ops guys looked into the white plastic bag (very briefly, I might add) but none of them could identify the contents either. Most of the smell, though, was coming from the clear plastic container that I think was a salad. It was red and orange bits in a foul green-black sludge. Eeew. And to think I work with adults....
Since I was on a roll, the expired cream cheese and yogurt containers went away, as did the large container of someone's leftover spaghetti. I never opened it, but the breadstick bits in the bag with it were rock hard so I assumed it had been a while. The stinking baking soda went too, since it was no longer absorbing odors but radiating them.
Then I wiped down the shelves, emptied the door shelves and carted them to the head for a good scrubbing - it took scouring powder and a scrubbie sponge to get that crud off.
Gah. Maybe it's just a woman thing.