I was hoping to get a response to my information request but also want you to have an answer. The most likely cause for this would be a critter. Often ground dwelling critters are under the surface and while we don't know they are there, your dog would. Often these critters do tend to be along the fence lines, under flower beds and around tree roots. Many of these critters are nocturnal so it does make sense that she is more obsessed at night.
Now they do make some devices that can help drive these types of animals away.
In addition, predator urine spread in that area can help keep smaller animals away and can also keep your dog away from that area as well. You can see predator urine here:
Unfortunately if it is a prey animal or critter leading to her behavior, the only way of correcting it is to rid the area of the critter. Training won't really do much.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.