If I’m in a room with people I know and I plan to leave the room for a long time, I tell them where I’m going. If it’s for a short time I do not.
What (if anything) is an appropriate time scale to leave a room without announcing when/if you’ll be back?
I’d say that it depends on whether you expect them to be there when you return, and/or if your presence is expected in the meantime for whatever reason.
Just have some situational awareness. Normally it wouldn’t be an issue to leave a room to get a soda from the fridge and then return. However if you’re in the middle of a timed game or while moving heavy furniture, then it would be rather annoying for the rest of the people if you just suddenly disappear without explanation.