As usual, the corridor at the farthest reaches of the dungeons was deserted. And as always, the door opened itself at the first touch of Al's knuckles: Snape had long ago tuned his security spells to admit Al immediately, whether he was present or not.
A velvet cushion-cover on the couch twitched, and the Compendium shrugged its way out of it, bouncing on the couch's cushions and riffling its pages by way of hello.
As it happened, Snape wasn't in the library, but the sound of bubbling and the clink of glassware came from one of the nearby Potions labs.