Glance having fallen out the window, its sheer inner curtain drawn against the strong light of the afternoon, Severus freezes. Impossi--well, highly improbable. There's an easy way to tell, though. A name no one, to the best of his knowledge, has used other than himself. If it draws no reaction, pax and pass, at least for the moment.
Without pulling aside the curtain, he opens the window, and calls, quietly, "Quirrin," as Q passes. His voice is still a rather dreadful rasp, but at least he has one.