How would my corn snake got out of a vivarium that was securely closed?

Do you have any screening at all? Are you sure there was absolutely no way he could have got out, maybe by a vent or something in his viv? Corn snakes are known for being little Houdinins. If he was still pretty young and small there are many ways he could have gotten out. If you have sliding glass doors for your viv, make sure you have little wedges so they can't push the door open. It's amazing how smart these little guys are. Make sure you look in every little nook and cranny in every part of your house if he could've possibly gotten out of the room he was in (though often they stay in the room they got out in). If you haven't found him, here are ways I've heard of looking for them: 1) Try the bait in a bottle trick. Get one of the big soda bottles that you get for birthday parties and cut the bottle kind of around where the bottle isn't sloping down. Then take the cap off (so your snake can get in) and put your bait in the bottle and flip the top portion you cut backwards and stick it in the bottle. This makes it so your snake can get in but has a hard time getting out. 2) If you feed frozen-thawed, put your bait in something that your snake couldn't get out of but of course can get in and put a heat mat under the container to heat up the bait's body to make the smell fill in the air. 3) This one may help you find him or may help you get the idea of where he's been. Put flour or sue powdery substance on non-carpeted flooring and if you see lines of wiggles through the powder, he has most likely been there. Now there are many ways of going about to find your little buddy but there are a few to help you out. Good luck if you haven't already found him.