Until you secure your pop door your birds are vulnerable. If you have a camera put it up because whatever it is will be back. My thought would be a weasel. Whatever the predator is it will be back until it has killed all of your birds. My coops and pens all have electric wires around them and nothing has gotten past the electric wires. Good luck...
http://www.poultrydvm.com/featured-infographic/tips-for-protecting-poultry-from-predators
http://www.poultrydvm.com/featured-infographic/tips-for-protecting-poultry-from-predators