Thank you for your post!
We do have reports of "Ghost Users" in some instances. We have found that it is usually caused by a damaged installation in which case you might:
1. Uninstall FileMaker Server
2. Rename the FileMaker Server Folder
3. Reboot the machine
This can also be caused by forced disconnects (such as crashing or hard shutdowns on the clients side). The resolutions become more broad here since it could be the clients handling of the application, a third-party plug-in, damage in the databases themselves, damaged install of the application, etc. Issues that you might not be aware of if clients aren't reporting them.
I hope this helps and if you have any further information please let me know!