Hmm, that seemed to work. It seems running the screen directly to the mini PC, and unplugging it from the HDMI connector leaves the desktop on as intended.

I was trying it running the monitor through a docking station and connected through the USB port to the mini PC, and that didn't work very well when I disconnected the docking station from the mini PC.

I guess you're right about the signal not beeing correctly.
( I was already running unattended )

Thanks!