It's not a plug I recognise TBH....
It may be part of a non-standard handsfree kit or something,
but as JJ says...it defo not OEM, so I would not worry about it.
But...if it bothers you, write to the previous owner (name etc
on the V5C logbook) and ask them if they have any idea or
knowledge of it. (They may need a copy of the last picture)