I'm just impressed that you have such a big garden. If a two foot hole appeared in my garden I'd fall in it!
Last year one of my dogs kept opening upstairs doors and rolling in my bed while I was out. I could never get in the house quick enough to catch the culprit (there is a point to this story) so I borrowed an cam corder and set it on automatic record on motion detection. Caught the culprit in the act on a priceless piece of film. Might work for your hole (just put the camera in a box with a plastic bag over it (with a hole obviously)) Happy hunting.