Dual monitors waking up but not going back to sleep

[Dave Ulrick]

Hi,

I have dual monitors configured on a Fedora 30 PC running XFCE 4 .14. 
Using the Screensaver (xscreensaver) settings GUI I've configured the 
monitors to be powered off after 15 minutes of inactivity. 90+% of the 
time the monitors do get powered off, but every once in a while I'll 
come back to my PC and see that the monitors are blanked with backlight 
on instead of being powered off. The PC is in a vibration-prone 
location--above the building furnace and near a street that gets some 
truck traffic--so I've considered the possibility that the mouse might 
move around on its own and wake up the monitors, but it seems that once 
the monitors turn back on they don't go back to sleep unless I first 
unblank the monitors by pressing a key or moving the mouse and then wait 
for the inactivity timer to turn them off again.

Any suggestions on how I might troubleshoot this?

Thanks,
Dave