Personally I would dowse for it.
Take two flags, of the type that is used for locating utilities, and remove the flags. Turn a 90 degree bend on each wire, about five inches from each end.
Note: you can use coat hangers, or any wire as far as that goes, but the locate flag wires work best for me.
After bending each wire, take one in each hand, and hold hands shoulder width apart. Hold the wires somewhat loosely, not too tight!
Start walking a grid of your area and watch the wires.
The wires will react to ANY CHANGE OF MAGNETIC FIELD WITHIN THE EARTH! Be it an electric, phone, water, gas, line etc., or even empty conduits, culverts, septic tanks, cisterns, etc.
This isn't magic or witchcraft. It is simple science.
When you walk over any change within the ground the wires will SWING either inward or outward. It does NOT matter which direction as each person can be different in their makeup and as to how the wires react.
For me the wires will turn and swing inward.
The wires will swing IN when passing over an object and then start swinging back out as you go away from the object.
Watch where the strongest point of crossing is and mark the spot. Cross the spot in several directions to "pinpoint" the object.
Then start digging!
Now keep in mind that this could be ANYTHING, but it sounds like you have the area narrowed down pretty well.
Give it a try and let me know how it goes.
Good luck!
get a probe and a shovel and get busy...any place where the roots have not been cut you can ignore....should be an easy task to locate a recently dug hole in a 15x15 area..just my thoughts
The probe is a good idea, dug dirt is usually loose after being put back in the hole. It most likely is close to a trail. I read a good post by another member last week that basically said stop, and relax, and study the area. Look for a hump or depression on the ground. If it covered with leaves, look for the leaves that arent so compacted. The dog is a great idea, my dogs get me into all kinds of things. I dont like to search in direct sunlight, early am or late evening. I would also set up a tripod, and camera, and take pictures. A camera will see 600 times more than your eyes. I take hundreds of pictures on my hunts. You will be amazed at how much you miss. Mangnetromity will help you find it, but its pricey. I know I just spelled that wrong. You also just gave the location away with your google earth photo, you most likey have several folks looking for it now.