Just thinking out loud and bearing in mind Colin's suggestion about asking the garage if it is an MOT fail, I suppose it is possible that the car could give out genuine AND false alarms. One way to test would be to deflate one of the tyres by, say, 7 psi (around 20%) and see if it activates.
But then I look in the manual to read that the system doesn't operate at low speed. Don't fancy testing this theory out on the motorway!