The find out which app grabbed your key you could use the debug keysym XF86LogGrabInfo; run
sleep 2; xdotool key "XF86LogGrabInfo" and then immediately start pressing that particular key, repeatedly then check for output in /var/log/Xorg.log.
Or use xdotool to press keys + XF86LogGrabInfo at the same time, see falconer's post here
