I think this is the typical voltage differential between the neutral conductor and the grounding conductors, if this is the case, then go find some super conductors that wont have any voltage drop and replace all the neutrals.
Kidding, I have run into this problem so many times with IT techs, just about any code compliant circuit will have some voltage drop between a neutral and grounding conductors, many of these techs will specify a voltage deferential voltage of .5 volts or less, now I have no way of getting a voltage level this low, unless you run #8 or larger neutral for each circuit, and if these circuits are fed from a sub-panel, and there is a voltage difference between the neutral and grounding at the sub panel, then you would have to up size this neutral.
The second theory that they could be meaning is they are detecting a high electro magnetic field, this can be caused when circuit conductors are not run together in the same raceway, common problem with mutable location 3-ways and 4-way switching, or boot leg wiring where load is using a hot from one circuit and the neutral from another, this would take turning off all breakers and seeing which breaker is using which neutral by taking amp readings on the neutral.
But the original mention theory of a grounded neutral is or seams the likely one, which one would just turn all breakers off take all neutrals off neutral bar, and see which one has continuity to ground trace it out and remove the connection.