Apparantly she DID see you. You probably shouldn't be spying on her. I could maybe understand walking by, just wanting to be near her. But you don't want to cross that line and appear to be creepy. If you like her, you need to let her know. If you are too nervous to come out and say it then find a way to drop hints and flirt a bit. But don't stalk her. That's no good. If she does mention that she thought she saw you go by. Don't lie. It's the perfect opportunity to be truthful with her to tell her how you feel.