Good post !
..... Newer farms that have only used mechanized farming techniques generally don't produce much....
Correct. Since the advent of mechanized powered tractors, there is less and less manual labor in fields.
...Western Europe and the North Eastern US maybe, ...
Yes. In England, for example, they can go into ANY FURROWED PLOWED FIELD, and have a chance at coins. That's because those fields have had 3000 yrs. of continuous cultivation. Not the same for the USA, where our history is only 300-ish yrs. old (of which the last 100-ish were mechanized). And on the west coast, it gets worse yet : Only 150-ish yrs. of crop cultivation, for the most part (Ag. didn't pick up here till the later 1800s, generally speaking)
So it is utter folly to simply go to "any random farm field" or "any random forest" and start detecting here in the USA. You need to know that something went on there. Eg.: picnic site, stage stop, home-site, or whatever.
Example: There is a cultivated row-crop area in CA, that I know used to be a mission period Indian rancheria . It fizzled by the late 1830s. And became row crops (sugar beats, etc...) starting in the 1890s. We go there to look for reales and buttons. We have ABSOLUTELY NO DESIRE to find modern coins or modern silver, etc.... But occasionally , yes, by accident, someone finds a wheatie, or a buffalo, or a clad dime, etc.... Those , of course, were lost during the modern Ag era. In fact, I even got a silver washington out there once (woohoo).
But as I looked back over all the years, and thousands and thousands of items found, and thousand and thousands of hours spent, I came the realization that : If it hadn't been for our primary objective (the 1790s to 1830's village site), then the occasional fluke wheaties and modern silver would not have been worth the time. Ie.: Even though, yes, a merc or wheatie "can pop up in the middle of nowhere", yet .... let's be honest: There are better places to spend your time. Do you really want to look for a week, to find a random single wheatie or silver washington ?