Correct it says fast track, but you, me and others who live in or travel frequently to BKK don't need to look. If first time arrival people with the head down and not looking like a tourist in Sukhumvit may miss the sign.

I always use the lanes for Thai passengers on arrival so don't take much notice of other lanes, but I have not noticed fast track signs being used on the far right of arrivals in D for example. Recently in D there were 2 lanes immediately to the left of the lanes for Thai passengers - possibly for Priority.

Recent poster may not be 'totally' wrong.