Where are WiFi passwords (WPA keys) stored?

[Robert Latest <boblatest@gmail.com>]

Not in /etc/wpa_supplicant/wpa_supplicant.conf, despite suggestions in
every bit of documentation that I got my hands on. In fact, that file
doesn't even exist on my jessie system. Nevertheless, when I
configured the WiFi network using some GUI tool in the XFCE desktop,
it worked. Even after a reboot, with no desktop running, I could ssh
into the system via WiFi.

So there must be a place, somewhere, where the WiFi passowrd ist
stored, outside the realm of some specific user. Where is it?

BTW, I did find a wpa_supplicant.conf file in some deep subdir of
/etc/dbus-1/... (I'm not at that computer at the moment). But that is
in some XML format that has nothing to do with the syntax described in
the wpa_supplicant.conf man page, and my WPA key doesn't seem to be in
there.

Thanks!