"they (either the perp or a friend who saw it happen) tell you after the fact "

If the thief told you that he stole from you, I would be amused by his report to the GM- "I stole from Bob, he didn't catch me, then I told him that I had stole from him, and then he had the gall to kill me! That is so unfair!"

I am not saying a GM might rule against the killer, but I wouldn't be surprised if the GM decided it was a non-GM issue.