When I was in 7th or 8th grade, I did just that while waiting for first class. I sat opposite a girl in the corridor and was staring right at her crotch, completely zoned out. When I came to, I realized what I was looking at and looked up at her quite angry face. I then, very smoothly I thought, blushed to a tomato red color and faked a zoning out with my gaze in another direction. Crisis averted.
Since she's asking Why is he just staring at me? I want to imagine a less hung-up world where she could add why doesn't he come talk or hit on me or something. Engage, damn you!
I've done this, lost in my own head gazing into my inner void while someone else on the bus wondered why I was staring at them. Usually, a sudden motion (say to check themselves or look uncomfortable) would pull me out and I'd look somewhere else.
I've made that yeah, I get that a lot apology.