I've noticed that in a meeting that has a room reservation, the room gets added to the meeting chat, and is displayed as "Unknown User". This can be very disconcerting for the other participants, particularly during confidential meetings.
Schedule a Teams meeting, and add a room resource.
Open the meeting in the Teams app, and open Chat with participants.
Hover over participants at the top of the chat window:
Number of participants includes the room.
Room is listed as "Unknown User".
Chat message: "<room name> joined the conversation."