Think I get what you're asking. You don't ont have to be booked, but you have to show proof in the form of boarding passes for each flight you tried to get on, that way they see physical proof that you were at the airport and made an attempt. Just listing yourself, or what I believe you mean by "booking" yourself, on a flight is not enough. Another thing that's been done is get to the airport, get the boarding pass that you receive prior to the one with a seat assignment, then go back home. So now even if the flight was wide open you still have that piece of paper they wanna see. However I strongly advise against this as you're definitely playing with fire. All they'd have to do is look at the flight load to see it had open seats. Number one rule in this industry: don't lie.