Can't find any way to handle with issue when user disconnects his account manually by going into Settings > General Settings > Connected Apps > Disconnect.
After user has beed disconnected i can catch an error only when trying to get something from his account. The question is if there are some way to ping/check Xero user's credentials as i'm going to implement something like heartbeat for this case (i did not find any kind of callbacks for this case which may be useful).
Thanks!