Once when flying out of T4 at LHR Dad Mum and myself were on a flight to BOS and Dad approached the F check in area he was asked if he was either Gold or in F. He responded that he was Gold (travelling in WT as it happened), he was also checking in Mum and that I was travelling with them though in a grander cabin than them! They didn't ask for his card but he usually already had it out so I guess they could have seen it.

I've been asked frequently (and asked for proof) if I'm in or joining the right queue, usually because I'm looking slightly like a tramp before I've been and had a shower and a facial etc.