I feel like girls are not obvious at all. Like no homo, but I've walked down the streets on my college campus and I know the dude in front of me is a good looking dude but the girl walking the opposite does not look at him at all and pays absolutely no attention to him. I know because I was following their eye movement. I know for me, if I saw an attractive girl, I would look at her and I think she would even catch me looking at her even if she doesn't know if I'm doing it just to do it or actually checking her out.
Most Helpful Guy
When there are others around that could see then the girl tries her absolute best to be really non-obvious in checking out a good looking guy.
Often it's enough for the girls to see the guy from far away. Completely unlike us dudes. And I read an answer here one time that girls can feel when someone is observing them, so you observing the girls made them nervous and thus they didn't look at anybody (I don't know if anyone else but you and the guy were around, but if there were other guys around, she wouldn't even look at them if she felt like she was being watched).
And yeah, I agree with Theguyoverthere. But I wonder if girls think of that as a "Come and talk to me"-signal ...I must ask that question here sometime0