I see a lot of post where people have snakes escape. They are good at it but if you take the proper precausions it shouldn't happen. I've only had snakes a couple of years but have never had one escape. I built my 3 main enclosures. I also have a couple of aquariums that I used for temporary housing in the beginning.
If you use the locking devices for the screens, I don't see anyway they could have unsnapped them. The ones I use are firmly attached to the screen and then hook under the plastic at the top of the aquarium. It's hard for me to unsnap it. You can't pull it up because it just makes it tighter in the plastic. You have to push down and then pull it away from the glass to lift off the screen.
If they are the right size and bent properly into shape they can not be removed by the snake inside. I was going to post a pic of the locking devices I use but I just found out the rechargable batteries are dead. . . . Again
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I think they were ZooMed products.
As far as it pushing his way through the mesh, I don't know what you can do about that. I never heard of that either but I guess it is possible.
Can you post a pic of your Grey rat snake? I have a wild caught Black rat and like him as well or better than my corns because he is more active (not aggressive) and eats anytime you offer him something.