954 Hi Ginger,
This can be a difficult situation if you can not catch the act. For now I would bury Trudy’s poop in the holes she digs to make it less tempting to redig into the spots. Technically, I consider this a housebreaking issue (even though it is outside the home) and I would troubleshoot first by looking into the “restless spirit”. If she is not digging to keep cool (in that case make sure there is a cool spot for her to go to in the garden) I would attempt to drain the prey drive somewhat before leaving her unsupervised through a game of tug, fetch, or tug/fetch.
Other then that there is no way to get around having to catch her in the act – which would require you peeking at her when she doesn’t suspect you are and using a proper “dog god”. If you can’t supervise her than ideally she is better off not in the yard until the habit seems kicked. This is similar to if you were house breaking in the home.
But, keep in mind, certain habits may be impossible to become habits if we are trying to fight mother nature. An extreme example would be a client of mine that wanted to correct their dog from licking objects, grass, etc.. Certain things are in the nature of the dog when exploring and interacting with the environment, so be sure to give a proper outlet for things like this that you want to prevent.
For instance a dog could be corrected for chewing shoes, but we give a bone as an outlet. A dog can be corrected for peeing in the home, but we give the outside as the outlet. Digging is similar – be sure to give an outlet or you may set your dog and self up for failure and frustration.