Looks like an old cellar hole that was filled in similar to one in my area. It's size would suggest a privy however. Based on its location to other foundations around it you can make a determination. Locate a larger house foundation and take a look out the "back door" area. If its far enough but not to far then I would say privy, if it's further out around old fence posts or other torn down materials it could be a cellar/ice hole to a shed of some kind that had no foundation. When I have found a privy there are normally more then one on some land my family has you can see where the outhouse was moved many times not to far from its original lactation. Most easily noticed by ground slumps were the ground sunk over time.
Either way I would say investigate the area well with your detector. I myself have found great coins, bottles, old signs and other like rusted treasures littered around such holes.