I will log this and if it's decided that it's working as designed, it will be closed, but the issue will be documented.
The server can be shutdown, disregarding connections made to the services manager. Example:

fbserver -a
gsec -user sysdba -pass masterke
GSEC> display
[The list of users is shown, with alignment problems, but at least shown
Now go an shutdown the server manually: it doesn't complain that there are connections. There are no attachments listed in the Properties dialog either.]
GSEC> display
connection lost to database
GSEC> ^Z