Thanks for comments: tires are run flats - winter style. Outside temperature was about -5degC. May have been some salty water on the road (the snow ploughs had been out. The lane was clear of visible snow.)
Brake temperature would have been somewhat cold due to outside temperature - had been on the highway for 40 minutes, using brakes only occasionally to adjust speed to pass. Hadn't felt anything out of the ordinary before this incident.
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