When we see someone we recognise in a crowd, we unconsciously raise our brows for a split second. When we see someone attractive, we unconsciously raise our eyebrows at him or her for a split second. This is unnoticeable if you don't look for it but clear if you are looking for specific signs.
This facial movement tells the other person that you recognise or have noticed them from the crowd. This can give the impression that you are friendly and open. However, be warned that if used on someone who is not attracted to you, then it can lead to confusion.