Oh I'm hip to all those sneaky things. LOL I've been playing Sims since the first one. I'm always in their inventory and look for the 'discard spoiled food' tab on them as well. I am extremely familiar with shelves (which even toddlers can reach to put dirty dishes ON but NOT to clean them up) and have triple-checked my dozen shelves. I think I would have noticed the green mist around a Sim, as that's the easiest way to locate stashed dirty dishes, but most of my Sims have the Forever Fresh trait so that's not the issue, Also, my morning routine for children is potty, shower, eat. For toddlers it's potty then change clothes. They get a bath after they eat.
The other reasons those couldn't be my problem are that 1- it's a brand-new house, they'd never stepped foot in it to leave dirty dishes on shelves, and I certainly didn't put any out when decorating. 😉
2- They couldn't have had dirty dishes in their pockets nor need a bath because they didn't get the moodlet until after they entered the house.
I did figure out what it was after a serious search on forums, and I'm very upset about that as well. For some bizarre reason fish in tanks now decay and the tanks DON'T give the green fog to tell you there's any problem. It seems that now you must name each fish to keep it from decaying (foolishness) which is a bit annoying since I keep so many fish stored for magic potions. Once I found and sold all 'foul' fish in my 4 tanks and named the survivors, all has been well with them.
But your suggestions are certainly all valid and thank you for trying to help. 🙂