Originally Posted by
Tafflyer
Short answer is that you'll never know.
I have often suspected that some agents (not only BA) mess around with seating for family, friends or other staff members. In one instance I got upgraded after making a fuss that my UD seat was changed between lounge and gate and when going back in flight for a look, did in fact see a number of crew around that seat chatting away very happily with the occupant. Circumstantial, yes. Surprised, no.
I was traveling home on staff travel from BKK a few years back and was offered a club seat but warned that there was an issue with the recline. When I got to the gate, The agent told me that the seat had been fixed by an engineer. I think in OP's case that the close out people were reallocating seats due to a reported defect and may have given that seat to staff.
I can tell you that BA gate staff don't have the authority to move people around.