I think the busking thing is a known bug. Ooh! Free guitar everytime you busk in San Myshuno! My Sim now has about a half dozen guitars I didn't buy for them.
Another Sim has a camera I didn't buy for her. Neithwr Sim is a klepto and swiped items tell you where they were swiped from if you hover over them. These don't.
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It would be good to post feedback on this AHQ EA thread that refers to the issue
http://answers.ea.com/t5/The-Sims-4-Bug-Reports/TO-ADD-Guitars-in-inventories/m-p/5680337#M29648
http://forums.thesims.com/en_US/discussion/817478/hello-come-and-introduce-yourself
http://tinyurl.com/OneRoomOneWeek
http://tinyurl.com/rosemow
My Showcase thread https://forums.thesims.com/en_US/discussion/948861/rosemow-s-rooms-showcase
The same thing happened to me (not guitars). My sim went to a festival, humour and hijinks, I believe. She brought bubbles and a snow globe (I know that, because I brought them). She had about 6 fireworks that I didn't buy and have no idea where they came from. This was between one loading of the game and another, in real life two days apart. She doesn't have the kleptomaniac trait either. I don't play rotationally.
The fireworks are the prizes if your sim was on the winning side (jokesters or pranksters).
Ah, well, that explains that. Though I didn't think she was. Thank you.