Severus does know that, and is just hoping that the problems attendant on getting to someone at Hogwarts are enough to defeat a whim--and that Slughorn has spidey-sense enough not to go to too many well-publicized parties. "I should think," he says, adding with a quirky look, "One does have to be careful not to lift pet hair from their clothes, though. Or someone else's hair, although that's less of a problem."
He nods silently again. Dreamless Sleep is a dangerous seduction to people with horrors in the shadows of their minds, but one needs the occasional period of rest free of nightmares as well as unpleasant REM cycles.
"Oi!" he objects loudly, covering his nose with both hands and directing eyes at Rus which, if they aren't really built for Kicked Puppy, at least manage Astonished And Surprised-At-You Kitten. "This is a perfectly normal nose, I'll have you know," he scolds Rus and embarks on a miffed rant*. "Gift of the Caesars, I shouldn't wonder. It's only you south-eastern ponces think there's anything unusual about it, which is a wonder given the school is in Scotland, and that's because the Romans weren't actually interested in anything below their border except as a source of taxes. Revenue and a new surname and maybe Bath if they had to be here anyway, that's all Londinium was to them!"
"All right," he accedes, amused, "but this time you'd probably better either hit his sleeve or aim for the board, and if you go for him twice he'll know you're targeting him."
* He's totally being whimsical here, in case that wasn't clear. ...But accurate. :D