Sims swiping objects!

Some sims might legit have the klepto trait as well so you might want to look into that. Also, the recycle disciple trait will cause sims to swipe your stuff if they haven't recycled in awhile. If those traits aren't on the sims, and you don't have the NAP active, check all neighbourhoods and repeal that NAP any time you see it active. Sometimes it will stick on sims and they'll use it outside of their neighbourhood. It's quite time consuming but you will have to log into at least one household in every neighbourhood to check. You can use the testing cheat to insta repeal the NAP off the mail box to so you don't have to actually play out the whole repeal process. Turn off NAPS in game too so that you are in control of which get voted in so you don't have to do this twice. ;) IF it's still happening (and it might), it's a bug unfortunately and you can add your "me too" to this one: https://answers.ea.com/t5/Bug-Reports/FIXED-ECO-NAPs-from-all-neighbourhoods-are-applied-to-visiting/td-p/9185155
The author of this thread is the one who's made a mod to fix it as well and he explains where you can find his mod should you need/want it.