Ok, the 14v or whatever it decides to be will have no current attached to it I expect so it is a high impedance imbalance.
When you ground all the stuff to a common, it will disappear and there will be no change in current draw.
Measure the AC current between the points, and I am guessing it will be in the milliamps at best.
When I'm faced with this situation, I use a sacraficial 1R resistor. I bought a couple of hundred 1R/0.25W metal film for $2 or something, so they're perfect. No risk of damage to my multimeter, and an instant "something's not right" indicator.
If that 15V is "leakage", then the 1R will remove it, the couple of milliamps will generate next to no heat, and all is good.
If the 15V is non-trivial, then the 1R will smoke, or go bang to indicate how much oomph is behind it! And for 0.1c it's real cheap insurance
Just my take on it.
(If you don't have 1R, anything that you have plenty of will do... 4R7, 10R, 100R, they'll all do the same thing for you)