The sign for "thief" is just a combination of STEAL and PERSON.
THIEF = STEAL + PERSON
You might see some people signing "bandit" to mean "thief." This sign is based on the idea that the traditional (old time) bandits had mustaches.
BANDIT:
Here's a version of the sign "GUNS DRAWN" that can be used to mean ""it's a hold up" or "rob." The sign uses modified "H" hands with the thumbs sticking up.
ROB / "hold up" / mug:
Notes: