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 don't announce it, and clearly nobody cares.
Just once somebody told me, confused, "I never seen you leave, but I am seeing you come here for like 5th time. Are you jumping out of the window?" (no), and someone else just noted "Yeah, I've noticed that too."