Wasp eater

Wasp weasels are small constructs created by someone or other in the service of Baron Klaus Wulfenbach. They serve the useful (to the Baron) function of identifying people who've been invaded by slaver wasps and thereby turned into revenants -- including the sneaky, dangerous ones who haven't been converted into shambling zombie-like monsters, but rather look and behave like normal human beings until The Other calls on them. The first time we see one, Klaus is using it to make sure that Bangladesh DuPree hasn't gone Revenant. (She hasn't, and she later points out via typically twisted Bangian logic that Klaus should have been able to figure that out purely behaviorally, but oh well.)

On this particular occasion, Klaus gets more than he bargained for, when the weasel discovers that the messenger from Sturmhalten who's calling on him at the same time as Bang is a Revenant (and is taken off to be, um, "treated"). We see from this the basic way the wasp weasel works: it sniffs the interesting person, and if he/she's clean, hums contentedly afterward, while an encounter with a Revenant provokes a loud "HISSSSS!", raised hackles, and so on. The reaction can be considerably more emphatic, ranging up to full-fledged freakout if a mob of Revenants shows up. (Or is the weasel's freakout on this occasion because Agatha is carrying the gadget that contains a "special" slaver wasp?  We don't know for sure.)

Klaus says the weasels "hunt" wasps as well as detect them. Never having seen a single, infectious wasp on the loose, it's hard to say exactly what this means. Presumably time will tell.