A couple of things that caused a stink for me were:
Potatos-Keep them in little cabinet next to sink and forgot they were they-smelled something bad for days before I tracked it to the cabinet
A wet facecloth under a towel in the laundry room-Wasnt bad till I lifted the towel.
Closed washing machine-for some reason if I close the lid it makes a bad smell if open its never a problem.
KC our cat dragged some food up into DD8 room and under the bed-I could smel it as soon as I hit the halfway mark on the stairs-kept opening doors and windows to air out smell took days to find it
Made a whole chicken and instant potatoes one time and put it back in the oven while eating and completely forgot about it--My house smelled so bad it made us gag--We called in landlord because we figured something had to have died and they could find nothing--About 7 or 8 days later I opened the oven to cook and found the pans-I nearly threw up it was so bad--We opened all doors and windows and threw everything in the trash and stayed in a hotel for a couple of days still smell was gone
Hope this gives you a couple of ideas as to where to look!