Also a good idea to check the magnetic field in the search area. In the past I have relied upon the USGS magnetometer stations or the calculator but today I was having difficulty hitting the target. I got out my smart phone and a magnetometer app and found I was way off on the magnetic field strength. I changed the frequency and it made a big difference. A thousand uT is plenty to throw it off. FInd an mag app that you can freeze the reading. Do it a few times to make sure you get a good average. BTW, the mag on the smart phones can be out of kilter. Take the phone and do a figure "8" pattern vertically and it will set the mag much closer. Of course you have to aim it to magnetic North to get a reading. Still probably not real accurate. I'd sure like to get one of those Earth field Mags in the link post #1.
So maybe my original theory is partially correct--the magnetic field in the search area can be affected by iron or soil mineralization. This is more prominent than a minor solar magnetic storm.
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