Kleptos will absolutely steal anything that isn't nailed down (or locked away). One stole my sims first accolades award right infront of him AND his friend. She didn't seem to mind that she had witnesses. There was nothing I could have done, the award was lost either way. I have no excuse though, I use the Show Sim info mod, and I should have checked. She never would have been allowed inside had I known.
If I can, I'll make a basement room where not even my own sims have access (no stairs or doors). That way I'll know that things I don't want to lose will be safe. I'll also some times save that basement (as room) to my library, just in case something freaky happens, like the game crash and I haven't saved properly in a long while.