Think of how many times you'd "know" it was her and it actually wasn't, and then forget that you "knew". To state that you knew it was her means that you would have taken any odds on a bet in that second. If I bet 100:1 on a dollar that it wasn't her would you have taken the bet, right?quite frequently. not always. and, yes (it sounds like such a cliche) there were times when i'd be at work, the phone would ring, and i'd know (by a feeling in my guts) it was her (note: i received many calls at work, at these calls came in at various times during the day).
My point is that you would only commit it to memory when you successfully guessed that it was her.