Oh we had such fun (not) when we had our last pup (Jack Russell Cross) We are fenced completely and surrounded by fields but we hadn't actually accounted for the fact that JR's can dig for England! Many's the time we have had to chase after her over the fields. She wasn't in the garden on her own either - if our backs were turned for a minute or two and she spotted a rabbit, a frenzied dig and she was off.
She is nearly 7 now and now so skittish, and, although we let her out in the garden on her own, I skill check on her regularly. However well trained they are they have their lapses and rabbits are just so tempting.