The only thing my Klepto has stolen is the Landgraabs cars! 
I had a legacy, (still working on it) and my first generation had the klepto trait. Only thing is, I never got a queue to "swipe something", whether I was at a community lot, the neighbors' houses, and she never stole anything but two huge streetlights from when she went exploring in the mausoleum (NOTE: She was an Elder. Ugh!). D< Grr... I swear, she must've visited every single house, and I monitered her. She didn't have the good trait or any traits that might have conflicted. She visited the art gallery everyday for about four sim weeks and nothing!
Needless to say, (mostly because klepto is my favourite trait... :]) I was quite disappointed with her.
Another point is, with all my other kleptos, I had to keep going into map editor every fifteen realtime minutes to replace the items they stole. So, is there a possibility of a modder being able to make a mod that, after every sim-day (or sim-week) and replace/fix all the objects taken by a klepto? (<--- not including active household.)
-Awfully Affy
