I had this happen to me on Tuesday. We were in Row 1 and therefore all bags must go up. We boarded about mid-way through boarding and the bins over row 1 & 2 were closed. BA has recently started doing this which I like, as it stops people walking past and putting bags in them (including those in Club Europe, but in row 3 and further back etc. It has worked well as we tend to board late and each time it has meant that there has been space above our seats. Anyway, we stowed our 2x carry on suitcases and then our backpacks were top and tailed so they only took up space as if it was 1 bag. All OK so far.
Fast forward to near the end of the flight, where I went to get my power cable. That's odd, my backpack was gone. Checked above row 3, nothing. Row 4, nothing.
The gent across the aisle in row 2 then "suggests" it may have been further back. Imagine my surprise to see it back at row 6.
Mrs FB then sheepishly mentioned that she noticed him moving bags around when they boarded (late), so that must have been when it happened. Clearly self-entitled so and so's and what was interesting after having seen the pax manifest on the iPad left in the galley, they had no status at all and weren't crediting to any OW program. So just a bunch of rude, clueless people.
Anyway, the FA sensing my disgust offered to store it in the cupboard at the front so that as we deplaned I didn't have to wrestle my way back to row 6 to get my bag, which had been correctly stowed above our seats originally before said over-entitled person decided his groups bags were more important than mine.