You may have detected in an old cemetery and not even have known it... I know I have. There was an old school where I grew up, the original building was built in the 1880's but had been replaced in the 1920's. My cousin went to that school and back in the 80's before it was torn down to make way for a supermarket we detected the place hard. I planned to go back after the building came down to see what I could find once the paved areas were taken up, and see if there were privy's from the original building....
I never got to though, turns out that whole area had been a cemetery before the school was originally built, and the graves "moved" in the 1870's. It seems though that all that was moved were the headstones and maybe a few of the wealthier families ancestors. As soon as they started digging there they started finding bones, called in archeologists from the local college, and I never got a chance to go back after they removed them all. After that I found out that a couple of other old schools in the town (which I had also detected) had been built over graveyards, as well as one small neighborhood on the edge of town.
So check your old maps, sometimes what was there before the building or park you're detecting around might surprise you