Happened to me a few times – the last occasion I can remember was both ways for a Christmas trip to Miami. I can understand why; a couple in their early to mid-30s, not wearing designer clothes, clearly travelling for leisure just before Christmas. Of course people fitting that visual profile do fly F/are OWEs, but the numbers must be pretty small so playing the probabilities game I’m not surprised we were questioned.
At LHR the two staff members were very polite. They just said “this line is for our Gold card holders, which cabin are you flying in?” At MIA it was a bit more brusque “we called rows 1-4 only, which row are you in?” “Row 4”, I replied. “Yeah, I’m going to need you to show me both boarding cards”. Didn’t really bother me – I’ve had far worse from TSA agents. Everyone was just trying to get where they needed to at a very busy time.