Thank you for your post!
There have been a handful of reports about "Ghost Connections" like this. It seems like reinstalling the latest update of FileMaker Server has fixed this in the past (make sure to rename the Server Folder after uninstall). You might also consider updating to FileMaker Server 14 or 15 as the "Ghost Connections" don't seem as apparent there.
There have been some instances where "Ghost Connections" occur due to improper disconnects due to force quitting applications or network issues.
I hope this helps!