I'm sorry you've lost so many chickens. You must be so frustrated!
You really need to go over every inch, when you're looking for an opening. Check up high on the walls and the roof, too. They're both good climbers and can also drop down onto a roof from a tree or other object. Could they be prying a piece of the tin back to squeeze through? Check all the seams, too. Make sure they're all secured. Also, look all around the foundation for any holes. Even small ones that can be hidden by a grass clump. Is there a possibility that it might be a weasel? They can squeeze through really small holes, especially the smallest of the species. Check to make sure the door isn't flexing after it's closed, allowing a predator to slip in through a gap, also.
You could definitely keep a trap set all the time. Sometimes people that house chickens inside barns build them a separate enclosure of wire to live in, if the barn is more open than the chickens need, for protection. That's an option, too, depending on your circumstances.
Good luck. I hope you can find a way to take care of this problem soon.