A dodgy edit of your pic (done on my phone!)
What I have circled in yellow is your 13-pin outlet.
What I have circled in red is an external plug into that socket….it may be powering a lighting board or something, I can’t see.
What I’ve circled in green is how the dust cap to the socket (yellow) holds the alignment plate from the plug (red)…..helps prevent the plug from vibrating loose.
All you need to do is lift that dust cap, then twist and pull the plug to disengage them.
I hope this helps. I’d be interested to know what is plugged into the socket.
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