what do you mean by "session ID"? the only property of your connection that is both visible to the Stratus service and meaningful to any other user is your peerID (NetConnection.nearID). they are cryptographically pseudorandom, unguessable and unforgeable.
I am referring to the 'peerID'. If this is accessible, then it is an obvious determinant of whether a subscriber is online or offline. The question then remains, has anyone out there used the peerID to determine a subscribers presence. Specifically, online or offline. The assumption is that if a peerID token exists, then the subscriber has to be online.