Some rooms have the _PUBLIC flag set on them, indicating that these rooms are "public" — that if you're in the area, it should be ICly fairly easy to know that people are there, and that it's okay, even encouraged, to simply walk into a scene there.
When someone walks into a public scene, it's polite to try to include them. Of course, sometimes the conversation that was interrupted will be a more private one, in which case people might choose to leave to continue it elsewhere, or change the topic — don't take it personally when that happens.
The command +public will show you who is in rooms that are set _PUBLIC. Note that if you are in a public room, +public will always find you, even if you're set Unfindable. If you need to keep your location a secret, you probably don't want to be in public.
Builders are strongly encouraged to set as many rooms _PUBLIC as possible.