The only time I recall getting a pressure warning on my mk3 was in snowy/icy conditions. And I suspect it was the road conditions which impacted.
If you checked the pressures when the tyres are cold, then I'd agree with others that unless there were other factors like adverse weather or terrible road surfaces, that it could be the tyres or the ABS sensor. I guess another possibility is that there is some other module which reads the ABS sensors which could be faulty? Not sure on whether there is such a module? But I recall on my mk2 there were issues with issues being reported incorrectly - it was a sensor which checks other sensors which was faulty. So, maybe it's just a false report - and not a faulty ABS sensor if that makes sense.
My money is on the tyre or the ABS sensor though...