Professor Quirrell grinned, then took out his wand and flicked it in the direction of the door. “Of course,” he said, “people who lead interesting lives take precautions more thorough than the dalliers. I have just sealed us in. Nothing will now pass in or out of this room—through the crack under the door, for example. And...”
Professor Quirrell then spoke no fewer than four different Charms, none of which Harry recognized.
“Even that does not really suffice,” said Professor Quirrell. “If we were doing anything of truly great import, it would be necessary to perform another twenty-three checks besides those. If, say, Rita Skeeter [...]
Which raises the question of what the last two Charms in the set of thirty are; but the clear implication is that the spell that would detect the presence of the Cloak of Invisibility is part of the “twenty-three checks” which he performs on other occasions in Harry’s presence.
Which raises the question of what the last two Charms in the set of thirty are; but the clear implication is that the spell that would detect the presence of the Cloak of Invisibility is part of the “twenty-three checks” which he performs on other occasions in Harry’s presence.