Have you ever noticed how your GF only catches you checking out another girl if the other girl is really hot? I tried it once, looking at the ugly girls instead. I got to the point of not finishing my sentences before she even thought something was up. But if some leggy blonde walks by, I can not notice a thing and still get "were you checking her out?"
bottom line: it's not that they caught you looking, it's that they saw another hot girl. So They're alot more aware of guys checking out other girls than they are aware of guys checking them out. Most girls think it won't happen to them, and only notice if it's blatant.