You're in the gym waiting for the school principal to make a speech and everyone is talking, the whole place humming loudly. Suddenly it goes quiet because you all thought the principal has arrived, yet no one told you to be silent. Everybody else notices the others being quite and follows suit. Everyone is silent for a moment, but after they realize no one arrived and no one is enforcing the silence, the noise starts again. This has happened to me in classrooms and at seminars.
lull - a temporary calm, quiet, or stillness
The air left the room
... is a metaphor often used to describe the sudden quiet when everybody's attention is drawn to someone or something. It sounds like you are describing the same kind of quiet, even though the someone is only imagined.
The air left the room for a moment, until we realized that the principal had not, in fact, arrived.