I also had this happen to a Sim of mine recently.
While at a Humor and Hijinks Festival in San Myshuno, one of the two Sims I had attend it had to leave and go to Work midway through it.
So they ran off to the lot's entry/exit point and went off to work, while the second Sim stayed at the Festival.
A few in-game hours later, the prizes for the hackathon were given out, and for some reason the Sim who had went off to work just came back on their own, far before their workday was over.
I take control of them to send them back, but I can't. (separate issue it seems)
But then I look in their personal inventory, and I notice they're suddenly missing a LOT of the items they had been carrying.
Not every item, though.
Some stacks of items had lost a portion of the stack, for example they had been carrying about 50 Sunsurfer Frogs, but suddenly they only had 1.
Some were completely gone, for example they had a SimRay from their days of working as a Scientist, but it was gone. As were all 20 of their Plasma Packs.
The items were all still there before they had left for work from the Festival, but it seemed like the combination of the Festival prize giveaway, and them being at work and leaving early on their own, could've been connected.
I kind of wish I kept multiple saves before this incident.
