A secret? Or ignorance?
When they arrive at the airport, they're holding hands in front of everybody. Even if their families and friends magically forget that, there were cameras. So why is it that people can't see they're in love? When they found out they hand been living in an island alone, didn't they think "Gee, is there a possibility something might have happened between them?".
I get that he was an anti-social and didn't like that she was so popular. But that doesn't explain why they were BOTH pretending like nothing had happened. If they weren't planning on dating after returning, why did they hold hands in front of the world?