How do I detect if a character has left/disconnected from a Room?

We have a tool for that, called ClientConnections.

In short, a ClientConnection is a special prefab that coherence can spawn for you when you connect, and destroys it when you leave. You can’t create it yourself.

Once spawned, you and other clients can query for these ClientConnection Prefabs and use them to indicate the presence of a player in the game.
If a CC disappears, it means that player disconnected. New CC appears: someone joined.

You’ll find more info here: