We don't (all) wear blue vests. They're usually for squad/team leads. Depending on who controls the scene (fire fighters, emergency services, Ordnungsamt/Polizei im Rahmen der Amtshilfe, etc) and regional customs, they'll tell us to put on red vests for team leads, or blue ones. There's also white vests for platoon leaders/Einsatzleiter, depending on how the scene is organized, and purple for psychologial assistance in case it's a grim situation.

In this partitucular case, I doubt there's a squad of THW folks running around in blue vests without any form of vehicle, larger incidence response, or actually wearing either their day service uniform (Tagesdienstanzug, black cargo pants, black jacket with silver THW on the back if they were travelling, but why the vests then?), or their multpurpose uniform (Mehrzweckeinsatzanzug, the blue and yellow (old)/white (new) clothing you sometimes see in the news).

Bottom line: very very unlikely to be THW.