Each of the hubs have a temp queue t_x with Subject/Queue = _$HUBALL, ID = discovery_server, & Address = Primary Hub IP address, what is it for.
Release : 20.4
Component : UIM - DISCOVERY_SERVER
This queue is used to communicate robot changes to the discovery_server. Things like removing, adding, or moving a robot, stopping or starting the service, and potentially communication failures making it seem like it is stopped.
Also changes to probes such as activating or deactivating, adding or removing will result in messages getting created in the queue.