Have you found any deep clad coins? There are some areas where coins get deep real fast. I didn't find any deep coins with my Garrett AT Pro until I used manual ground balance. Used coins program, iron completely rejected, sensitivity 7 in STD mode. This set-up gets a loud beep on a dime in air test out to 10"
Using it in a nearby '70s park that's been detected 100s of times I got a high tone every sweep with #s 80-85 (dime). Got a clad dime 7" deep in hard, dry & somewhat mineralized ground & it was loud. Probably had sat there 20-30 years.
If you're AT Pro was made before July & hasn't been updated, it probably should be sent in. If you have the AT Pro with the camlocks, it should be good.
Some places have been worked over thousands of times, other places are renovated with fill dirt. If fill dirt has been used, trees that should have roots extending outward will be buried. If they're showing on the ground, likely no fill dirt (at least not around the trees).
Some possibly helpful sites are county historical societies, maybe the map sites like
www.maphistory.info Also interesting, Wikipedia for ghost towns, existing towns, much info. Also
www.hometownlocator.com has current maps, sites of former schools, post offices, churches, etc. There is a seller on eBay that has CDs of state & county history books, many thousands of pages for each state. Best wishes, George (MN)