You made up an entire backstory for a stranger in three seconds. It was probably wrong.

Your brain hates an incomplete picture and fills one in instantly from clothes, posture, one overheard sentence. It feels like perception and it is mostly invention, assembled out of whatever you have seen before. The speed is the giveaway. Nothing that fast could have involved any actual information about the person.

Catch yourself doing it once this week and write down the story you invented, then notice you have no evidence for a word of it. Strangers do not need verifying. You just want to hold the story lightly enough that meeting them can change it.