Bandwidth while waiting

I’m thinking about a room device that is open and waiting for a long time for others to connect. Does this used any bandwidth while idling and waiting

Nope, after the last person that leaves the media can flow for another 20 seconds and then the media session is terminated from the focus and the only thing that will be running is bosh connection(signalling) to the server with pings every 10 seconds (few Kb).
This use case sounds like you want something like: https://github.com/jitsi/jitsi-meet-spot