I suspect that it is a blocked seat. It could also be that there are two people travelling together and they are on separate tickets. You will know one way or another when either no one comes or else someone comes who knows the person in the window seat. I cannot imagine why anyone in their right mind would deliberately request a middle seat.

Far far more irritating is when, as happened to me once, when someone tries to sit in the E seat in CE.

"Oh do you mind?" said the person (I had something on the seat) "My friend (wife/partner/ tarte de la nuit) and I are in different rows"

"I'm sorry to disappoint you" said I " but in this Cabin the middle seat is meant to be free. That is what I pay for"

"Oh. Well maybe you'd swap with her then?"

"Well maybe the person in that row would swap with you?" I replied

"He doesn't want to"

"and neither do I" said I heedless of grammar

With that he knelt on his seat and said to whomever

"She won't play ball"

"No "she" won't. "I retorted "This is British Airways and not Turkish Airlines. We do not play with balls in a cabin"

The man behind me, who clearly had been been taking more than a passing interest, roared with laughter and that rather deflated Mr 1F. He sulked all the way to Madrid watching his Ipad. My indifference threated to overwhelm me.