The other day, I was stared at by a smiling girl. It feels strange.

From what you said, you don't know each other. Don't think too much. Seeing is mutual. Maybe you are looking at her, and she just smiles back at you. Maybe something about you attracted her to stare at you. Maybe you look like her acquaintance, but you're not sure, so I'll look at you more. Looking at you with a smile shows that the woman is polite and friendly, and it is normal to be looked at outside. There is nothing strange.