rbulsara: You're definitely right that the potential should be very very low. But the assumption that "there is no current between the two rods through earth" definitely simplifies things a bit too far for the purposes of determining ground potentials.
There's definitely ground currents flowing in medium-sized industrial plants or generating plants. Actually, there's plenty of utility-induced ground currents flowing in open farmlands due to the common practice of many utilities to utilize the earth as a ground return path -- this is bad enough that cows can detect their step potential when they drink water and will in fact refuse to drink their water rather than receive a shock.
I realize the cows are somewhat off topic, but not all that far off. It's common for even small residential or commercial systems to have measurable ground current flows due to poor installation practices, aging motors, etc. In large industrial facilities, the problem is even worse due to induced currents, zero-sequence harmonics, etc.
Instrumentation and communications engineers have long been aware of the problems ground currents can cause on their systems, and those problems are the reason that isolated-ground receptacles are in widespread use (I have serious doubts about the effectiveness of IG receptacles, but that's definitely a separate topic). This is also the reason that the shields on shielded communications wiring is often terminated at only one end -- terminating both ends could lead to such high currents flowing on the shields that the cable overheats -- on a long cable run, that definitely implies a substantial voltage drop, too.
I certainly agree that if a ground mat is well installed and cared for, and relatively new, that the voltage measured should be fairly low, probably less than a volt. But it would not surprise me if larger older installations that have received average to mediocre care develop substantially higher voltages, say in the 5 to 20 volt range.
50 volts is kind of high, I admit. But my guess is that this would not be an abnormal condition at some facilities.
jghrist makes the very valid point that measuring any voltage would be extremely difficult. Similarly, the effects of any such potential would typically be negligable, at least to safety and the operation of power equipment, except under unusual circumstances. That's part of the reason that ground currents are typically discussed more often than ground voltages.