all is not lost!
PROBE! he had to dig a good size hole. we locate filled in wells with a yard long spring steel rod probe. we go along poking the ground and dug out or filled in dirt will most time let the probe go down in the dirt. it takes a little umph pushing but see directions below. we can probe the whole outline of the well or privy, and then dig away.
to make a probe, find an old style car with a trunk lid that does not have a shock absorber--old 60-70 models--there are two spring steel rods that cross under the trunk lid, right under the part closest to back glass. using a couple strong folks take the trunk lid off and get the rods. be carefull. heavy lids. rods are real flexible spring steel. take a one inch pipe about a foot long--steel, drill a hole in one side in the middle. insert the rod, weld it. you now have a tee-handle. grind the other end of rod to a rounded point. you now have a flexible probe you can bear down on and at an angle and try to push it in the dirt. go along and push it in until you find the disturbed dirt.
roll up the sleeves and get the shovel. we recently dug out 3 shallow wells this way--located with the probe and a nice cache of bottles. my bottle digger friends are never without one, and the MD"ers use them too for trash piles, privys etc to locate, dig out and detect the throwout dirt.
good luck hh carey