What is that SMELL?

I was away on business last week. When i opened my car after coming home it smelled like rancid feet. Great googley-moogley it stank. What is that smell? I spotted my forgotten lunchbox. Carefully i opened it - empty; no smell. There were some trash bags on the floor - empty; no smell. I remebered that it had rained with my window opened a crack. I checked the upholstery - no mildew; no smell. I checked all the way under the seats to see if the was an errant banana peel - nope; no smell. I checked the glove box for nesting mice (a problem we have been having with work vehicles) - nope; no smell. I checked the trunk for dead hobos (don't ask) - nope; no smell. Nothing IN the car smells, but the inside of the car smells! I left the windows open this weekend, but the smell was back this morning. UGH!