If you understood how the system works, that would make perfect sense to you. Unless all five plugs failed at once, there should have been plenty of advance warning that things were going downhill.

The fault detection system works by comparing the current drawn by the #1 GP to the combined current drawn by the remaining GPs. If none of the plugs draw any current, there is no imbalance, therefore no fault indication.

Clearly, the intent of the fault detection system is to warn of a problem that is not otherwise apparent. I pay attention to the light every time i start my car. If you want to test your glow plugs before every start, have at it.