Adrian saw Lex in his costume earlier as he was heading down to the party, so it isn't a surprise to him anymore (and that's probably for the best, so he doesn't openly stare in the middle of the party), but it still draws his attention easily. Then again, it would be hard not to, considering he's managed to keep Lex within sight for most of the evening.

He is not hovering.

He is now, however, watching from across the room, and when Lex turns his head, Adrian raises his glass at him and smiles.