
I know the feeling!! I have a 200ft secure garden, and it has always been safe for the Collies. When I took on the Chi's about 6 years ago, I was horrified to see, as I looked out of my bedroom window, 2 Chihuahuas chasing next doors Collie round it's own garden!
After scrutinising every possible escape route, I found that it is very easy for Chi's to use hedgehog runs! After blocking this, I never knew whether I had jailed the hogs in my own garden. But, funnily enough, my dogs don't come in the house with ticks on them any more. So I guess they are resident in someone else's garden now!