It mainly happens on the bus. Me and my boyfriend will be sitting on the bus together and a girl will sit directly opposite us, despite being many other available seats. Then they proceed to, not subtly, look over and stare at my boyfriend. They either constantly turn their head, lean forward a bit and look at him or they just stare for a while, then look away. It doesn't matter if I'm sitting closer to the girl or not, whether I look her right in the eyes or not, she'll still stare back! They've stared at him so intently before that they didn't notice me looking right at them.
I always thought it was a rude and odd thing to stare at someone who's sitting next to you but some girls can't help themselves. I know they find him attractive,