Yes, it would seem that you have a very slight leak between the -ve and the bonding.
When you say that the MASSE - light doesn't come on, have you observed it in the dead of night with no ambient light?
I can give you an illustration of what I had once.
I noticed a faint glimmer, only observable at night on the MASSE -. Couldn't see it in the daytime.
By a process of elimination I found the cause.
It was the AC power pack used to power the laptop.
Because at that time I still had the Calpeda Air con pump, the bonding was also connected to the AC ground through that pump - standard Amel.
The AC/DC power pack for the laptop must have had a high resistance internal connection from DC-ve to AC ground, because when I unplugged the AC connection the MASSE - light went off, similarly if I disconnected all the USB connections I had between the laptop and the nav instruments, the MASSE - light went off.
BUT I could only see the MASSE - light in the dark....daytime it looked fine.
In a previous boat I had a similar issue with a negative wire that was wet....no real connection to anything, just damp.
Something like that could be the cause of your circumstance.
In Paul's case, it seems clear to me that it's something on the powered side of the nav instruments cct breaker that is causing his issue.