Parktronic and two red lights

[jberks]

crawling in traffic last night and as the parktronic cut in I noticed that initially the two inner slim lights came on as normal, then they went out and the 4 red lights came on without the usual preceding yellow ones (and I was too far away from the car in front to trigger it anyway). It happened every time I stopped over a5 min period, then after a further motorway blast, it hasn't happened again. I assume its some form of fault warning. Any idea what it was trying to tell me. Mucky sensors?