I've had "phantom voltage" readings occasionally in situations like you're describing.
A house that was originally wired with Knob and Tube, that had very few outlets at the beginning, will have new outlets extended off the K&T. As the years pass and outlets and load are added, the service gets an increase in size and specific loads are broken out and connected to individual branch circuits. Most of the outlets extended off the K&T remain as originally added.
One common extension method around here has been fishing in BX (pre-drain wire AC). The source end would be made up where the hot and neutral were available off the K&T, most of the time, in an accessible joist or stud cavity. A junction box was rarely used. The metal armor, stripped back a foot or so, should end at a "birds eye" fitting, but commonly a couple of wraps of friction tape was all that was done. The black and white from the BX were then soldered to the K&T and reinsulated. That's it.
Now, if the BX got fished in along or over another grounded surface, say old iron water, sewer or gas lines, the casual contact will fool test instruments into showing a ground.
I've had the runs that aren't in any contact with any ground show a capacitve coupled voltage of 50~70 V. The higher the input impedance of my tester, the higher the voltage it reads on the sheath. Bonding the sheath or the outlet box bleeds of the capacitive charge, and everything is fine.
One can get the same effect with BX or AC extensions off of pre-ground wire NM.