Hidden twin??

Hey have any of you lovely people had a very early scan that hasn't detected that you were having twins? I know I might sound crazy but I had a early scan at 5 weeks, where we could just make out a tiny dot but since then my HCG levels have been pretty high and my pregnancy symptoms seem to be stronger than ever before. So I just wondered if, even after a scan, people have gone on to find out they're having twins?