To find out which app grabbed your key you could use the debug keysym [`XF86LogGrabInfo`][1]; 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][2]


 [1]: https://lists.freedesktop.org/archives/xorg-devel/2011-June/023508.html
 [2]: http://askubuntu.com/a/406907