Detached displays don't restore when you start MC. They do remember their positioning and size (including fullscreen) when you close and re-open MC, but you have to manually detach them again first.
You CAN do it with a script though.
I have a system that runs a detached display on our Video Wall at the office, and I have a script that auto-loads MC on startup, starts playing a particular playlist, and then detaches the display. It works great for if the system needs to be rebooted.
To Detach the currently active display:
mc17.exe /MCC 10037, 1
To re-attach the currently active display:
mc17.exe /MCC 10037, 0
To toggle back and forth:
mc17.exe /MCC 10037, -1
It does work even if playback is stopped.
You can make any of these commands Zone specific by appending a Zone Index to the end of the command after a colon. The Zone Index is zero based (so, zone 1 is :0, zone 2 is :1, etc). So, for example, to send the above Detach the Display command to the second defined zone, you'd use this command:
mc17.exe /MCC 10037, 1:1
Lastly, the Detach Display MCC command described above ONLY works for currently running copies of MC. So, if MC might not be running already, you need to launch it first. If you just need to launch MC first in a script, simply call the mc17.exe command by itself in your script or BAT file like this:
mc17.exe
mc17.exe /MCC 10037, 1:1
(Note: Don't use the mc17.exe /Power command line option because that will close MC if it is already running, like a power-button toggle. Just call mc17.exe by itself.)