Do you play rotationally ? Does this mean do I play sims on different lots? If so, no, I only play one family per save (except for a save that I use only for building, but that's a different save file, not this one). I edited some other lots without the Sim traveling (IOW, from manage worlds) - nothing major though, and no cheats.
If you do, did you have a flea market happening while you were playing another family ? No. My Sim did go to the flea market, and bought all the snow globes. IIRC, that's not when he lost all the other stuff.
What were you doing when you noticed the stuff missing ? After a vacation in GF or S ? Did you travel with your Sim ? No vacations. The first time I noticed it (when everything but the snow globes disappeared), the Sim had traveled - maybe to GeekCon? Or to the park in Oasis Springs? I restored from a previous save and I noticed the elements were missing after he'd gone collecting in the area around his home lot.
Are you playing an kind of active career ? My Sim is in the Tech Guru career. His roommate is in Painting.
Which items (type) do go missing ? Only collectibles or other stuff ? The only stuff he had in his inventory were collectibles, the snow globes he'd bought at the flea market, and plumbing parts. The first time, everything in his inventory disappeared except the snow globes - so frogs, crystals, elements, plumbing parts, fruits, all of it. The second time, the elements disappeared for sure, and I think some other collectibles too.
Like EuphorialQueen, I have used the Bring Here option, but I think that was after the problem started. The lot traits are Fast Internet, Home Studio and Haunted - does the Haunted lot trait cause stuff to disappear? I have the save files if that helps.