What follows is the proof. For the proof, I assume that you know basic electricity. Ohms law, resistor dividers, and the general properties of transformers, capacitors, and inductors. You don't need a great deal of this knowledge, but enough so that you can picture what's going on since there are no pictures in this post. To find the source of this stray voltage, we must eliminate sources that don't fit the evidence.
Possible sources are induction, resistive coupling, and capacitive coupling.
There are other possible sources, but the odds of these being the cause are so remote that we can ignore them. If you put a resistor between the ground probe and earth ground, and measure the current through the resistor, you will measure up to about 40 uA. The voltage across the resistor will depend on the value of the resistor, according to ohms law. The voltage measured between two ground probes is zero, regardless of their position or orientation. Let's start with induction, since that is the common myth. If this were happening, then electrically the tank would look like a transformer.