I have this issue too (along with various other items appearing in sims inventories too). The instruments mostly seem to appear in the inventory of played in-active sims that have been used as NPCs to busk, but this is not the only explanation, as children are never busking in my game, yet they often have several (adult-only!) instruments in their inventories. Not sure if clubs are playing a part (I have children playing instruments in clubs, but haven't noticed yet if they play other instruments than the ones available on the lots). I haven't found instruments on teens/adults that haven't been busking.
Sims who have been used as NPC vendors at the flea market, often (but not always) have the collection items or toys they were selling in their inventory when I rotate to play them.
Sims have books written by other sims appearing in their inventories. I have no idea where they are getting them from, as all books my sims have written have been published (no copies ever made), so these books are never available to me in the game.
I made one sim as a partner for an in-game born sim, placed him in an apartment and didn't know at the time, that you now need to actually play them to mark them played (they used to be marked played automatically when placed in a home). Before I got around to playing the sim, he had been used as an NPC stall vendor at festivals, but never as a flea market table vendor. When it was his turn to be played he had a couple of posters and 5 snow globes in his inventory. Snow globes have never been sold at the flea market so far in my game (probably just a matter of time though), and none of my sims had started collecting snow globes at that time, so no one had duplicates to try and trade at the flea market either. Unless NPCs have started collecting items (and I doubt that), I simply have no clue why that sim would be having snow globes in his inventory, when I hadn't even started playing CL households yet. The boxes that you find snow globes in hadn't even started spawning in my game at the time (the globes he had were not the ones you can buy at swag stalls).
