Bulb out indicator on dash?

How does the ECU decide if a bulb is broken or not before it lights up the bulb out indicator?

I have a false sporadic LH main bulb warning and that happens to be where I take the trigger for my light bar relay, so I suspect that has something to do with it.
The light bar is wired with a SPST relay powered from bat+ and triggered from the main beam wire, nothing strange there at all.
It seems to work fine during summer but now that it's colder it lights the bulb in the dash every now and then at startup, never after that. The bulb out light goes out if I turn on the mains (and light bar) for a few seconds...

So, does the ECU look at the resistance at the bulbs or is it more complicated than that?
Would a relay with a flyback diode help?
Or a relay with a specific resistance?

The CECM sends a limited current and monitors the load. If the current is too low, it triggers the fault.

I also have a random "bulb is out" error for front left parking light. It comes on about every 30th cycle of ignition. The bulb is OK, connectors are tight and clean, and yes, I have tried swapping the bulb.... It sucks. I suspect there is some corrosion or oxidation either inside the headlight, connectors or the wire itself.

Ok, seems like we have the same fault, I disconnected the relay trigger wire from the main beam wire and the fault is still there. I soldered it back on and checked the wiring at the head lamp but couldn't see anything wrong.
Cleaned the connectors and still had the same bulb out light...

It's time for inspection and I really don't want to show up there with a yellow warning light on the dash, even if the lights work.

Sorted!
Turned out it wasn't a false error at all, there was a bad connection at the bulb. This issue was on my Skoda and the bulb is a H1, as to the H7 on the Golf. The retainer spring didn't ground the bulb well enough.

Lucky you. Seems my Lucky Coins have ended on this issue

I'm glad there was a fault that I could fix, I really don't like seeing warning lights coming on without reason. Hate that...
Thanks for pointing me in the right direction, I was sure it had something to do with the aux light relay