[ The 'bubble' is new. Sam doesn't panic at the sudden shift in perception the same way other people might, he's been thrown around by unseen forces often enough that he's bracing for an impact that doesn't happen based on instinct alone. He's considering whether or not he could break out of it, most magical effects outside of those from Astoria herself don't seem to stick well on him, but he hasn't tried actively forcing the effect yet, but the voice temporarily distracts him from those thoughts.

He does try to turn to face the figure he can dimly see moving across from him. He doesn't recognize it, not yet, but he is trying to squint at the face. The man seems calm enough, so Sam will respond with a similar level of neutrality, since he would really rather not get into a fight unless he has to.

The real answer is that his first gut instinct was that something was weird about this guy, but once he had Sam's attention he'd realized that something strange was going on here. ]

You were kind of suspicious. People around here have been dropping dead under suspicious circumstances. [ No huge leap of logic needed there. ] I know that how you people do things around here isn't really any of my business, but I'm not gonna stand by and let someone get murdered in an alley.

[ Obviously the fact that it could have led to him being murdered in an alley hadn't been a sufficient deterrent. ]