This is the moment Humbert has been waiting for. He explains to the reader (whom he addresses as "Gentlewomen of the jury" (28.1) that her purity is important to him, so he will only take advantage of her while she sleeps. (Uh...)
He wanders downstairs looking for a drink, goes to the men's room, chats with the concierge, and goes out the lobby to stand outside in the dark. The anticipation of returning to the room is overwhelming him.
A voice out of the darkness addresses Humbert and begins asking all sorts of familiar questions about Lolita. The man's double entendres suggest that he is on to Humbert, but both play innocent.
Walking back through the lobby, Humbert is caught by the "blinding flash" (28.24) of a camera.