I want to run a script before I log out of my GNOME session, to warn myself if I have forgotten a pendrive plugged into the machine.

However, all solutions I have found have not been enough for me:

• This similar question suggests editing the /etc/gdm/PostSession/Default file, but I don't have the permissions to do that. (And the file is currently empty on my machine, so there are no existing hooks to take advantage off)

• I also found this post suggesting a workaround but, according to the first reply, it does not work if I shutdown the computer instead of only logging off.