Penny's plan was simple.
To catch if there was a mole. Thus, the three of them just idled on the island. Penny, Shawn, Slater, and their three hostages stared at the sky until a helicopter finally came into view. They tried guessing who was inside the chopper, with Penny even wanting to put a bet on it.
Obviously, neither Slater nor Shawn agreed with the bet. So, in the end, she forced the other three hostages—including the unconscious one—to have a bet with her.
And just like that, they started counting all the aircraft that passed by them before sending their own.
To do that, Penny devised a plan to make the chopper fly on its own without anyone in it. Using some extra gauze, they wrapped all the handles and buttons. But to start it, someone needed to be inside and get it to the sky.
So, Shawn volunteered—or rather, Slater forced him to do it.