Jackson did this to me once last year; we were at a dog park we had never been to and it was empty, just us there. I'm sitting on the bench, letting him do his thing, and he sees a squirrel and goes chasing after it.. well there was a huge hole under this part in the fence, like he easily got under there and he's 15lbs, and he just took off as if there was no fence there at all and continued chasing the squirrel. I saw it and was in disbelief, as I hadn't checked the perimeter of the fence yet or anything. Luckily, once the squirrel was out of sight, he came running back! But whew, gave me a heart attack! I used to let him off leash a lot more since we live on 3 acres unfenced but once he turned a year old, that prey drive of his got strong! He's becoming more reliable with his recall- I've been working so hard with him and I think it's really THE most important thing you can teach.
I am so glad Mya is okay!