Sorry mate, I did drop one the other day, they tend to linger - recall that Seinfeld episode? That was based on one of mine..... 


Probably is some vermin....last year a smell like you described started in my last car. I figured it was something the kids had eaten in the car and left to rot under the seat. My lovely wife volunteered to find the offending suspected sandwich....

Later she said 'Thanks a lot!' and looked rather angry - turns out as she reached around under the seat she found something soft and goey and the likely culprit - and pulled out a rotting mouse!

Better her than me!