Ok, so THIS just happened in my game... I play with different household, and I have an alien-sim who is level 9 in the scientist career, and she has a lot of stuff in her inventory related to her job.
But I was playing with another household, with a sim-gardener and that sim-gardener went to the Casbah Galley, and in the street in front of the gallery, near the food vendor, a random townie was selling stuff on a flea market table THAT BELONGS TO MY ALIEN-SIM!!!!
The alien-sim was not on the lot, I was not playing with her, the townie is not related to her. So I noted what was sold from her inventory: several serums, her SimRay, an electronic part, a DNA sample and even her potion of youth! (There were also snow balls on the table that don't belong to her)
So I went to play with my alien-sim, and sure enough, all THESE items specifically were gone from her inventory... But the rest of her inventory was still there.
I think this will give you a big clue about why items disappear sometimes from inventories.
I had this happen in my game a few days ago, but I got lucky and found the items in the household inventory. This definitely needs to be fixed. In the meantime, I'm moving things out of the personal inventory of my Sims into their household inventory when I switch households. This really hurts rotational players.
Same here but on Xbox One.
I only started noticing this after parenthood + kids room stuff have been installed, since I usually don't like keeping much in my sims inventories I rarely check, but I started because of the voidcritters to often go into the kids inventory...
