It's hard to be subtle when not only are you affecting your entire environment, the only other creature, and owner of the establishment, is a similarly ethereal being. And one that takes great care in maintaining his shop, of course.

The atmosphere changed the moment Aziraphale sensed something divine. Tension, confusion, which certainly don't change the moment he turns to look past the shelf and finds himself staring at Azrael, without a trace of familiarity or recognition.

But that's certainly an angel. A powerful one, as far as he can tell. But he looks entirely out of place, and he can't remember to ever have seen him. He would have heard about an angel this powerful, before he had ever been assigned to the gate.

So, he stares.

"....what--" He stumbles in his words, stops to try and gather himself. "I'm sorry, who exactly are you?"