My experiences with this;

1) Return leg KUL-LHR in F after holiday with my wife. I was hospitalised on day of flight so no showed, wife returned home (at my insistence!) I didn't check-in & despite 241 voucher in my name apparently no questions asked and certainly no come back.

2) My wife recently returned from AUS in WTP after a holiday with a friend. Her friend (who's 241 they were using) had to stay in States for work. My wife mentioned to check-in agent that she would be flying alone & again no issues or comeback.

My conclusion based on limited sample of 2 trips in last 18months is that they couldn't care less as they've got your YQ & one less mouth to feed or a seat to fill with stand by pax (caveat: both no-shows at outstations)