maybe a mouse died??
I get bottles under the couch or seats of my car and WHEWIE THEY STTIIINNNK!
The smell is different than a mouse. More like old milk or food of some kind. One time I found a 1/2 eaten gallon of ice cream under one of the girls beds with 2 spoons in it. I got them together and said to them, "Why didn't you invite me to your ice cream party?" That was too funny to be mad at them!