What does it mean when a girl stares at your face while standing next to you?

This girl I know keeps doing that. She may not look at me when I talk to her (she is "busy" with her phone) but when she stands next to me and I don't say anything, she looks right at my face and I just can feel it (I don't turn to look at her). Why is this? Is this normal? She doesn't seem very invested when I talk to her (she answers, but that's it) but when I'm not talking she will look at me all she wants. It doesn't creep me out but I know some people might find it weird.
What does it mean when a girl stares at your face while standing next to you?
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