I recently noticed that Kreg and Nicodemus will occasionally steal equipment from the Avatar between about noon and 6PM (or well, they do this in the DOS version. I couldn't get them to in Exult).

Do any other NPCs do this?

I've heard this about Nicodemus before. Apparently it can be game-breaking too, depending on what he steals. From what I understand, the only way to fix it while keeping the game winnable is to use cheat mode to make him drop all his items.

Not sure which other NPCs are able to steal from the Avatar though.