Recently moved into a new home, built in the 40s. Many old 2 prong receptacles. Prior renovations with some new grounded three prong circuits, ground + gfci in kitchens and bathrooms, etc.
All of the three prong outlets have a working ground when I tested them with a multimeter. One outlet does not. I opened up, it appears to be wired with modern 3 wire including a ground wire. This breaker for this circuit is in the sub panel that was put in during the renovation with the other circuits that have updated grounded outlets.
I don't know what to do next to figure out why this outlet isn't grounded. Part of me just wants to throw a gfci receptacle in there and not worry about it, but part of me wants to fix it just for the sake of fixing it.
You need to trace (or correctly guess) the exact path of the circuit cable back to the panel so you can check for loose connections perhaps at other outlet boxes daisy chained along the way.
You may find a two piece tone generating tracer kit handy. You clip the toner onto a wire and use the hand held sensor like a stud finder to trace the path of that wire (and other accompanying wires in a Romex cable).
When cleaning anything including baby diapers, it is better to sweep off or scrape off as much solid and semisolid material as you can first. (1) Less cleaning fluid or detergent needs to be used, (2) Less of a mushy mess which in turn means that bleaches or disinfectants will be more effective.