Why would a girl watch me as I walk past when she was having a conversation with someone else?

I only know this cause that person she was talking to came and told me. They said her eyes just followed me as I walked past. I wasn't even looking at her. They said she didn't move her head but her eyes just did a sweeping motion of 10 to 2 as I walked past.
Why would a girl watch me as I walk past when she was having a conversation with someone else?
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