Possible glow plug relay stays on intermittently. Battery disconnects are cheap. Some things are initially so hard to find alternatives are a temporary solution.

In your case I would run a bulb from the glow circuit temporarily inside the car to monitor that circuit. They can be intermittent so the bulb should stay off. I would not trust the normal glow circuit indicator bulb as the indicator. The relay can malfunction I suspect and not light the normal indicator bulb.

Actually this is not that much of an unusual problem on these cars. Lots of cases have occurred over time.