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"So, has anyone here ever seen gaydar in action in real life, in situations where clues are not obvious?"
Yes I have, on more than one occasion. I knew a straight woman in the early seventies, who always knew when she encountered anyone who was gay, even if they were not sure about it themselves, and I repeat always.
I don't think she really picked up on specifics, she merely had a sixth sense of some sort and could 'read' people, for want of a better word.
So, from my experience, it's real. To have it to the degree that June (her name) had it is very rare, but I have seen various people over the years with various degrees of gaydar.