My other mystery smell - rotten potatoes...
Our house reeked when we came back from a 3-day weekend out of town. Problem # 1 was solved when we discovered the HVAC vent full of pee.
Odor # 2 smelled like moisture and mildew, strong and nasty, like like a bad leak! I searched high and low, to no avail. After a week of looking, I caught a strong whiff in the kitchen when the A/C came on. I was standing next to a duct adjacent to my kitchen island. Bingo! No, it wasn't in the duct...Inside the cabinet next to the air flow are my potato and onion bins and there, two rotto potatoes and two rotten onions.
Problem # 2 solved. Sweet, sweet, now I can think of invitng company to my house.