I think things have gone a little awry in this discussion.
The positive and negative sides of your battery bank should float i.e.NOT be connected anywhere to the bonding system (the green/yellow wires to the zincs, sometimes erroneously called "ground")
So, with your multimeter on VDC you should measure close to no volts (no potential) between +ve to the bonding wire, and -ve to the bonding wire.
The fact that you read +25 VDC between the +ve and the bonding wire tells you that, somewhere, the -ve from your battery bank is connected to the bonding.
This is what the MASSE lights tell you....either +ve is connected to the bonding or -ve is connected to the bonding, somehow.
The fact that when you flip the navigation breaker to OFF (it's a double pole breaker and breaks both +ve and -ve) the 25VDC between +ve and bonding goes away, tells you that the -ve is connected to the bonding somewhere AFTER that breaker.
So, this means that on some piece(s) of equipment powered by that breaker there is a connection between -ve and the bonding.
As Bill said, this can often be a "meticulous" electrician taking a wire from an instrument "ground" connection to the bonding.
A lot of instruments have their "ground" connection internally connected to -ve and that will cause this issue.
It could also be that this "meticulous" electrician took a wire from the metal case of some piece of equipment powered by this breaker, and connected it to the bonding.
You need to identify each piece of equipment that is powered by that breaker and check to see if there is a wire from it connected to the bonding system...when you find it (them), remove it.
None of your electronic equipment should have the "ground" connection connected to the bonding. The bonding is for electrolysis prevention, and is connected to all the metal parts in the boat that are in contact with sea water. It is NOT electrical "ground".
Hope this helps