Not really having permission, is when you get permission but not from the owner. In other words many farmers lease and to not own the land they work. So asking a farmer if you can metal detect his patch after harvest, is not good enough. You must do your research and ask the real owner. Case in point Mount Herman School in Northfield, MA. Much of the School's land holdings in the meadow and  by the river is posted No Trespassing !! They & some trusts, leases land to farmers and if you want to hunt here,  you need permission from  land owner and not the farmer.