Why is there a positive body voltage when grounded, but none when not?
I purchased a cheap multi-meter which also have only 200 and 500 volts ac (alternating current) settings.  These settings are much, much too high to measure the low body voltages of more or less 1 volt to 3 volts.
But your multi-meter will have very low dc (direct current) settings which you can use, if you were to convert the ac of your body voltage to dc voltage.
Connect a diode in series with your/the +ve of the multi-meter, which will convert the ac to dc which is then measured on the 200 millivolt dc scale/setting.
+ of multi-meter}...................k(diode)..........connected to earth peg.
-ve of multi-meter....................................probe in hand

Your multi-meter will now measure very low body voltage/s in dc, instead of (as a function of) ac.
A diode can be purchased from your radio repair shop.  I use a (1N4007) diode.