My (immigration) fast track passes were taken off me at BKK, both when entering and leaving. They looked identical, the one I was given on the incoming flight had no flight information, but the one I was given at the F checkin on the way out had the flight number and date written on it by the checkin agent before he gave it to me.

I ended up in BA Club World (business) on the way there, and was given a "premium" fast track pass on the aircraft. The man in the seat next to me was not, so when I was walking into the premium lane he asked me about this, and was not pleased to find I had one and he did not. I suggest waving his business boarding pass, and he encountered me later at the bag claim and said it worked but he had been told, firmly, that he really should have one next time. The crew on that flight were disorganised in several other ways too, their overall service was cheerful but a bit cack-handed.