There's a bland lack of recognition for a couple seconds, then a little click of ah, he knows who she is.

And by the lack of any other change, it doesn't matter to him that she is the archangel.

He nods slightly - yes, he heard you, no, he doesn't care to greet you back.

Right before it would be horribly awkward, he looks deeper into the library.

"I keep a catalogue." And perhaps a hint of 'if she messes with his organization, there will be problems between them.' also surprise, he has a very deep voice.