As a dedicated thief, I have noticed some interesting things about the thief-catchers, i.e., the Flaming Fist in Act 3 who show up after a shop-keeper is robbed.
First, they seem to come out of nowhere originally - I break into all buildings as a matter of habit and I never see extra Flaming Fist just standing around. But when they are 'called', they seem to come out of somewhere just off-screen and head to the scene of the crime.
They are always in groups of 3, although exact composition varies. Their names are generic, "Flaming Fist {some rank/title}".
Second, it isn't the same group each time. I know this because they stick around after appearing the first time. They seem to have rally points they fall back to and the rally points are sometimes out in the open. So more and more Flaming Fist squads show up and hang around as you steal more.
Third, they are worth XP.
Which leads to some consequences I haven't explored yet because attacking Flaming Fists can cause uninvited consequences:
If you kill them, are they resurrected? Or recreated?
If so, do the replacements have more XP? If so, it's a way to grind. (I know for a fact that killing named Flaming Fists is permanent and you only get XP from knocking them out once.)
If they stay dead, can you then steal from the shop-keepers who call them with impunity? Because generally the shop-keepers who call them will not accuse you themselves.
I will eventually test all of this myself, but I thought I would put this out for any murder hoboes eager to test a potential recurring XP pool.
