Funny, something similar happened to me yesterday. I would not say that people can actually "clock" me because I present very androgynous (no makeup, unisex clothing), so they either gender me female or try not to gender me at all. But they usually do not stare at me - yesterday was different. As I was walking through shopping mall, somehow my eyes for a brief second met with one good-looking woman who was dressed conservatively but with a definite touch of fashion. Next thing she was doing, was staring me down and up again (almost like checking out my shape) and with my peripheral I noticed that as soon as we passed each other, she turned around to take and extra look. It was kinda annoying, because I am usually very confident in public and those those stares pretty much never happen to me. And the same thing happened today, without meeting eyes - another woman turned around to give an extra check.
P.S. On the other hand, I have passed to small kids a couple of times (and then again, sometimes I did not

).