Proof of Yellow Fever Vaccination Required?

I'm have about a 10 hour layover in Panama City while flying from Peru back to the U.S. in which I was hoping to go out of the airport and explore the city briefly since I've never been there before. Do I need to show proof of yellow fever vaccination before I'm allowed entry, though? I'm not vaccinated against it. And any advice on what to see or do in like 5 hours in Panama City (if that's at all advisable)?

Thanks!

http://www.who.int/ith/chapters/ith2012en_annexes.pdf
Required in Panama if traveling from a country with risk of YFV transmission. Peru, yes. U.S. no. If you were flying from Lima to Panama only they would ask you for it (WHO card) in Lima at boarding (the airline) but technically you are flying to the U.S. so it is not required and if you exit the terminal in PC for a few hours and reenter to my way of thinking who would know or question.
You could go the Casco Antiguo for a while and wander around, unless you have seen enough colonial, but I understand with the on going metro construction traffic can be a nightmare in Panama City.