In your EMS dashboard, top right 'Manage Widget', enable Endpoint Management widget. In the widget, you can click on 'Unmanaged' device, which you can see a list of previously joined device that has been disconnected. You can see their 'last seen' status there.
It is likely that those endpoints are offline for long and exceed license timeout of 30 days, EMS auto release the license. If endpoints are in 'unmanaged' state, you can just let end user to manually input EMS IP or Invitation Code to rejoin back to EMS again.
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