Well two thoughts go into my mind on how this could have happened. Lets say you installed the unattended on a laptop months ago, the laptop has been sitting unused so it was no longer connected. When the client finally turned on the laptop boom it's back and by this time it looks like a random new session.
Second, is that you may have left the installer on a pc and someone just randomly clicked on it again to see what it did, this installed screenconnect and connected to your server.
In my opinion this is why the software need some form of ACL list that can block by the GUID. This way we can whitelist only sessions we want and block anything else.