I have a gtx 980 with proprietary drivers with one screen on DVI, and one on HDMI. When I invoke lxsession-logout via clicking the icon on the right side of lxpanel, the logout dialog asking if I want to shutdown, reboot, suspend, logout, etc, appears on the other screen that does not have lxpanel.
I'm not sure if lxsession-logout is responsible for such weird placement, it might be some setting of Openbox or even a bug in Openbox itself, but definitely when you click that button the window should appear at the mouse's monitor side. And lxpanel (which also is involved) does not have any possibility to position windows of applications which it starts.
Thanks, I pushed a possible fix in git, let me know if it fixed the problem for you.