When a client that is part of the presence set disconnects from the app, due to the app being closed or the network going down, the particular client will continue showing as 'present' in the presence set for about 15sec.
During these 15sec, the Ably library would try reconnecting the client back to the platform. If it succeeds in doing so, the client would never have left the presence set. However, even after 15 sec, if the client couldn't be re-connected for some reason, then the client be out of the presence set.
This is in contrast to the client calling the `leave` or `close` methods, in which case, the client would immediately leave the presence set.