99.9% of the time when this kind of issue occurs it is due to a poor, or intermittently bad grounding (earthing) of the headlight bulb. What happens is, when there's a poor earth connection, not all the current can flow to ground by the normal route (through the bulb's earth wire) and what can't will try and flow through the next easiest route (path of least resistance) to ground, which just happens to be through the main beam's positive supply wire, via the warning lamp which has its own ground/earth point.

The dimming of dip beam is due to the poor earth (high resistance) and the warning light glowing is a symptom of the resulting leakage to earth. I'd assume the reason it only does it sometimes is that the poor ground is intermittent, so look for a frayed earth wire, melted bulb holder or corroded connections at the bulb/holder. Also try running a clean earth direct from the bulb straight back to the battery negative to see if that cures things.

Occasionally a faulty H4 bulb (as a result of vibration) can do the same thing as both main and dip filaments are close together and any distortion of the filaments, internal supports or insulation can cause leakage from one circuit to the other. Try a fresh (decent quality) bulb.

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