Just thinking back. The first false alarm went off after I had driven, roughly, 200 miles after picking the car up from the dealers. The second one, this last Sunday, was a good 200 miles - probably more - since the last calibration.
In short, hours after recalibration. As I say, the only common factor is high speed driving. Also checked if car pulling to one side, it isn't, also checked heat of wheels for any possible brake binding, there is none.
Weird. Not overly fussed but it is a bit annoying.