Snape was just beginning to think that the bird had flown, when... there! That young blond. The way he moved. It was so subtle, so subliminal, it would have been difficult for Snape to describe the exact cues. But that didn't change the fact that he knew. The way that man moved was wrong. He hadn't lived his life in that body. He'd made the mistake of choosing a form too different from his own.
Snape knew the man was a lie, a figment of Polyjuice. And with that certainty came movement. The situation was supremely dangerous. The Minister, the Head Auror, the cream of the wizarding world was right here in this room. Voldemort could cause incalculable damage if it came to an open duel.
Snape was determined that it would not come to that.
He left his post against the wall for the first time that night, beginning to thread his way through the throng, silent, stealthy, wand out but hidden in the wide drape of his sleeve. Behind the bland mask of his own Polyjuiced face, his attention was riveted with a duellist's intentness on his prey.
The young blond with the barefaced bollocks to be chatting up a Hit Wizard.