I'll wager that some small critter died in your HVAC system or near the air intake.
One time, I had to swerve on a country road to avoid an accident. That took my car right over a dead raccoon in the road which not only took out my speedometer cable, but splattered all over the exhaust. I ran it a few times through a carwash that sprayed the undercarraige. Helped a little, but the smell of death lingered for a few weeks.
Speaking of tomfoolery, back in undergrad, we picked up a fish at the market and put it between the mattress and boxspring of one of my fraternity brothers. It was hellish and he went nuts, but he never found the fish until the end of the year when he moved his bed out.