This has happened to me twice. I work in a store and I was on the till when I was serving a lady, I scanned her goods and was putting her goods in the bag when I looked up to see that the lady I was serving was staring at me. She felt very embarrassed I could tell because she looked down and smiled almost apologetically. This had happened to me again today.
I felt self conscious at the time which led me to ask this question
Some just admire the beauty of others. If I am staring at another female it is most likely because I find her pretty and admire her. Yes, I am straight but it does not mean I can't appreciate how beautiful others can be. Now, if you get other girls staring at you with a nasty look, they are probably jealous, just know you have what they want.
Sometimes if I'm trying to figure out why I know someone, or if I like what they're wearing. or jealousy.