A good probe with a ball bearing on the end (a knob) is by a massive degree the best tool to find a privy, a metal detector may help but if there is metal that close to the surface in a hole you will find it with your probe, detectors are good when it's shallow pockets of rubbish that are hard to probe.
sure a Dectector will help,but will also help find the garbage pits that were also used for bottles/other household garbage,.........
whe usin the detector in a celler hole type are/house area - where your hittn alot of junk targets in on concentrated area--DIG!!!!!