Draco did notice her; he noticed everyone that entered the bar, whether he knew them or not. His eyes did linger on her longer than on most, though, because she was familiar. He knew who Chastity Montgomery was, of course; knew what she did for the Dark Lord, and had a decent idea about where she stood in the general hierarchical structure of the Death Eaters.
What she was doing here, he didn't know. It wasn't unusual for a Death Eater to be here, considering it was one of the pubs frequented by the upper class of wizarding society (as if Draco would have gone anywhere else, at least not without some other purpose in mind than an evening out). But she didn't seem to have anything in particular she'd come here for, since she was looking at him with interest.
He was too far away to speak, and wasn't about to yell to her, but he lifted his glass as if in toast. She could take that how she wanted: simple recognition, or an offer for a drink. He wouldn't turn down the company, if she was in the mood.