With XFCE, xscreensaver wakes up and prompts for password

I know this is an old thread but disabling Presentation mode in xfce4-power-manager applet fixed this

Since I am new I am unable to add a comment and ask you if you are using XFCE4. I had this same exact problem and tracked the problem to xfce4-power-manager causing this exact same issue.

Taken from the Xscreensaver FAQ:

Starting in early 2016, I began receiving reports that a program called xfce4-power-manager occasionally loses its mind and decides that it's very important that your screen never, ever blank, and it does this by simulating fake KeyPress events hither and yon. I don't know why. Your best bet would be to kill and/or uninstall that program.

Killing xfce4-power-manager solved my issue that you were having but I have a second issue of DPMS not working which you can see here: DPMS Stopped Working Arch (nvidia drivers - xfce4)