I always check. Those of you who are cabin crew will see me (Y class, window seat) reach under the seat to check at the point in the safety briefing where it gets mentioned, and if Mrs WHBM is seated alongside, you'll see me check for hers as well. Not just checking for absence, but also to get a feel for where it is exactly.

And always ALWAYS double check its location at London City, to know the quickest hand movement to get it out, as the engines power up on departure, and on short finals when incoming, for obvious reasons .....

Never found one missing yet, but did once (NOT on BA) find a spare one on the floor, which it turned out had rolled forward from the row behind.

I believe one of the demographics is amateur sailors going to their boats on the Med at the start of the season.