spontaneous logging out (gnome-flashback)

[Adam Funk]

A relative's computer is running Ubuntu 15.04 (installed as 14.04 &
upgraded twice).  He uses gnome-flashback with metacity, but I think
he's had the same problem with gnome-flashback with compiz.
Occasionally, erratically, it suddenly makes the drumbeat noise &
drops back to the lightdm login screen.  (This is especially annoying
in the middle of Minecraft.)  I can't find anything in the syslog
other than a normal logout, & there doesn't seem to be any pattern in
when it happens.  This doesn't happen every day, but definitely
hapened in 14.10 (I think it happened in 14.04 too).

I'd be grateful for any debugging suggestions.