What makes a person truly leave a room?

I was thinking about this the other day, standing in the corridor after my shift ended. One inmate — I won’t name him — used to sit in the same corner of the yard every afternoon, reading. Not much else ever changed. But when he left, the whole space felt different. Like the air had thinned. I’ve been wondering: is it just absence, or something more? What do you notice when someone truly departs?