I'll try my best to keep this short, so let's dive right in. One of my gripes with popular dating/seduction advice has always been stuff along the lines of "If she smiles at you, plays with your hair, holds a sign that reads "FUCK ME NOW, YOU STUD", you should maybe approach her and say hi" because that kind of thing never happens to me. Women smile at me when I'm a customer somewhere and that's it.
However, this semester I happened to have a pretty random selection of lectures together with a girl. I just noticed her at some point because I happened to see her over and over again. One day, when we left the lecture hall, I thought she smiled directly at me but quickly dismissed it because she was obviously just smiling at something her friend told her and happened to look in my direction that moment. Forward a few weeks, I was walking to the lecture hall and she was sitting outside, alone, and smiled the moment she saw me. I smiled back but didn't say anything. Then I began to wonder if she was actually trying to "invite" me or if I was just over-interpreting friendliness. I imagine there's nothing worse for a woman than simply being nice and polite and friendly and some guy misreads your signals and you have to deal with some douchebag hitting on you. Probably why so many girls put on a "resting bitch face" in public.

Anyway, tomorrow's the last day of the semester and there's very little chance I'll see her again next year. And despite knowing better - those obviously weren't IOIs - some tiny part of me will always wonder "what if"? I guess my point is if you think someone might be trying to encourage you to talk to them, give it a try, at least you won't be wondering "what if". Just be prepared to make a graceful retreat if you misread the situation.