could that be an option in the truck software setup that was enabled by default but the option was never actually put on the machine?

personally in my years of working in this industry i have never seen a lifttruck with a battery water level indicator sensor built into the battery from the factory, those were generally aftermarket addon accessories and were never tied into the truck system, they were independently monitoring battery water and had an led light on them telling you when the electrolyte was low.

of course nissan trucks i haven't worked on alot so i may be wrong about those trucks ;o)