Number plate light failure is unlikely to be detected when it is off as the detector measures load to the lamps.
It is most likely a brake light bulb. I had a similar problem but when I checked them they were all working ok, but after the warning kept coming on I changed both bulbs and the warning hasn't come on any more.

 

Brake light bulb failure is a different warning light.

The other bulb failure that is easy to not see is the rear indicators as there are two each side, so at first glance all look like they are working, even though one bulb has gone.

 

Hi Guys . . . plus David C & sunnydene who responded to my "bulb failure" problem.
This issue just isn't going away and quiet frankly it's driving me nuts. It's like it's got a poltergeist or something . . . one niggling little problem after the next.
I replaced offside and nearside number plate lamps completely including bulbs and I'm still getting the "bulb failure" warning.
I've now ordered a complete set of rear bulbs . . . that's stop/tail lights and indicator bulbs . . . offside and nearside.
Visually all the lights that should work appear to be working, so this is bloody annoying.
The warning is intermittent and appears shortly after pulling away, or after a short distance and then goes out again (then reappears). The "beep" is getting on my tits.
This is in daylight . . . my lights aren't even switched on.
The only light that isn't working is the 'seat belt warning' lamp - but for the life of me I can't remember if it was ever working in the first place - maybe someone can definitively clear that one up for me because I've just about had enough?
Regards

 

Indicators flash fast when a bulb fails - no dash warning . (But not if it's a wing bulb failure)

There's no seatbelt electronics to detect seatbelt and the seatbelt light is coded out so shouldn't appear on 99.9% of TTs.
If it's coded back it'll just appear for about 10s and turn off.

The only bulb warnings are on headlights and brake lights.

Not 100% sure on this but dont think fog lights give a warning.

Regardless the Only warning that "should" be available with all lights off should be brake lights.

I would recommend you change the brake light bulbs but when they are out use a flat screw driver head and give all the contacts a good scraping . Contact pins and the ground sides.

Dremmel or sand paper or just some metal to scrape some fresh contact surface up.

Maybe best to have a go on all the bulb holders in the rear lamp cluster.
The metal gets oxidised over time and maybe resistance is borderline.

Using some isopropanol contact cleaner would help and I'd also use that on the main connector and fit/refit a few times to ensure that the wiring plug has a good contact too.

Hope this may help.

 

I would just like to conclude my "Bulb Failure" warning saga by posting an image of the culprit.
It shows one of the double filament brake light bulbs.
You can clearly see how one of the filaments had detached itself from the top of a pin and then reattached itself further down. The bulb still worked but the change in resistance was enough to cause a "Bulb Failure" warning.
The warning no longer appears after replacing all bulbs and giving the connections a good clean.
Thanks for all the advice that led me to spot that one.

 

Good Spot. What a weird failure but makes sense at least you also know the bulb fail warning is working as expected and telling you where it saw the issue.

 

Not really a weird failure. Been running Golfs for the past 30 years and it seems that all the double filament bulbs fail the same way - when only one of the filaments detaches from one side. It makes intermittent contact to give you a false sense that it is connected. I found the easiest way to tell is to pull the bulb and look at it in decent light. If the glass looks like it is darkened on one side, it's a failed bulb that managed to re-attach itself (for a little while).

When I got my TT, I got that light warning on the DIS screen. Since I got it without a manual (would have been fun to read it in japanese), I thought the 'bulb out' symbol meant the car was imminently going to blow up. Since the DIS was orange and not red, I thought... "It's OK - I can drive a bit further". Then when the bulb finally went, I also wound up replacing the brake pedal switch.