Metaphorically she'd read through a phone book just to find out the name of who she'd seen. The world is a pretty big place when you're searching for what's literally a face in the crowd. Some would say the whole situation was absurd, but she knew this day had been in store for her. The more important a person is to become, the swifter one might be to realise. This time it took one blink in a mass of people, maybe less than a second of just the other face paddling to or from the trains at the station. And she knew. This wasn't a case of love on first sight or anything as dramatic; she clairvoyantly knew she had to find this face again. There was nothing as clear, hadn't been since. She couldn't even identify emotions she had for the person - might they even be important.
It was dead simple. The most important person in her life had suddenly passed her; something she had long awaited. And this person, now permanently engraved onto her soul, might never pass her again.