That is an indicator that quite high current..........higher than normal......has flowed
It might be that the main ground cable to the engine is "not actually ground" anymore, either corroded inside the cable, or whatever that could cause a partial open from the battery NEG to the block
Have you added any high current accessories? Electric fans, or a big stereo?
You must understand, that unless additional jumper straps have been added, there is a VERY poor ground between the engine and body. That is the original purpose of that small ground from the battery to body
Originally, that was to conduct current to "body" grounded stuff such as lights, heater, radio, etc.
Again, something has caused abnormal high current flow.
