Reasons a girl may stare?

Hey all,

I just wanted to say that i'm not in a place of considering "dating" this girl as the sub suggests but an interaction on Thursday had me questioning whether something is up and all you dating people might be able to give me some relevant insight.

Alright, I (19M) started working at this new library job like a week or so ago and there is this girl there of a similar age that has been friendly to me as a normal coworker would. I notice that she's looking at me 90% of our passive encounters (she walks by, parking lot, etc.) but I am like 90% sure she has a boyfriend in the first place.

This past Thursday, I was working within her vicinity for most the day & we chatted here and there. At one point she left for break and returned, passing through the library aisle that I was in (note that there's like 3 more ways she could've gone too). As I was working she stopped in my aisle and looked at me for a few seconds. I did a double take, a bit confused, looked back for a few seconds and asked "yes?" she said "sorry, i'm staring" and asked me to explain to her what kind of work I was doing, so I showed her. Other than that, most of my interactions with her until Thursday have been quite passive besides the looking.

People out there, what logical reasons might explain encounters such as these? thanks!