Every time you cross paths with a happner in real life, their profile shows up on your Main Page if your preferences are mutual. Whether you've seen the person or not, happn considers that you’ve crossed paths with someone when you walk past each other in real life.
Please note that happn also shows you profiles that are in a range of 120 km from your position, so that you can still enjoy the app to the fullest if you don't have many happners in your area.