As someone who has done plenty of sound recordist work, it’s known as ‘room tone.’

Also, I feel seen because I’ve had to explain that so many times. Even to people who really should know.

Terry Pratchett writes about this, how there is a difference between the sound of someone not being there and the sound of someone hiding and not making any noise.

He often writes about how things like bird song can be a type of silence and how a train that always passes at the same time every night, not passing at that time, can wake you up from its absence.