I'm so glad to hear that Guinness is back inside safe and sound. Luckily none of my cats have ever made it outside. My front door opens up into a hallway so my RB Pepper and Sky have run out there a few times but I could always catch them easily.
Maybe you should put Guinness in another room with the door closed before you open up your front or back door again. Hopefully he's learned his lesson and won't try this again. He may have thought that you were playing with him though. Good luck in the future.