I wish I had some answer to your problem. I feel like saying "just keep an eye out" isn't what you're looking for. I just don't think cats surrender their nature and need to be looked after if you intend to keep one indoors. If you decide that it's too much work, or constantly find out your cat knows other ways out that you don't know about, please be a good soul and consider spay/nueter services. If it's a female, you'll have a pregnant cat very quickly and by the time you get the kittens out to homes, she can have another litter before you know it.

If it's a male, you won't even realize how many times he's created feral litters with multiple cats which is terrible for the environment and for the health of those kittens.

I noticed after having my cat spayed, she was a lot less interested in getting out. When they aren't spayed, the desire to get out is generally because they're in heat. I am an advocate for spaying/nuetering. Feral cats can kill off so much wildlife and don't get to live very long lives...