I have read about this annoying occurrence on many Sims sites but hadn't experienced it until the last couple days. You go on vacation and when you return your Sims keep getting moodlets about something rotten or bad smelling in the house. You search and search for this elusive stench and never find it. Your maid cleans and never finds or cleans it.
LOL!!
My Sims by some lucky chance asked another Sim to move in and that Sim had the 'Neat' trait. She kept wanting to clean the house, even after the maid was there. I finally broke down and had her clean the whole house. I have not gotten that negative moodlet since she cleaned.