How did it know I had a towbar (after 300 miles)?

[V12noise]

Fitted a 3rd party harness for the towbar which includes the CAN module. All the trailer lights worked on completion, out the box, and I was very chuffed...

But when I hooked up the trailer and reversed, the rear parking sensors were on constantly. I thought ok fair enough, I didn't get the BMW unit, saved some money and would have to put up with it... but after about 300 miles of constant towing, I had to make a U-turn and as I finished the turn I heard a beep from the screen and it showed the diagram of the car with a towbar symbol at the rear. Now, whenever I go into Reverse with the trailer, it knows and I have the towbar symbol displayed with the Front sensors active. Great, but how did it work it out after 300 miles....??