I've seen no instructions stating that the recognition stars should be applied to motorcycles. They are sometimes seen on captured motorcycles.
This bike seems to be with Polish Armoured. They may have interpreted instructions in another way or simply been more impressed by the American way of doing things than the British were.
Don't know if the British had this habit, but many Canadians ordered to paint white stars on their vehicles would paint them on crooked out of protest so as to not be mistaken for Americans. I originally thought that was the case with the Polish bike, but it's not crooked enough :-)
One of the cute ladies fueling up the bikes has a white star on her helmet.
My wifes late Uncle Raymond once told me that in the run up to D-Day and for weeks afterwards, he spent almost all his time painting stars on all the British vehicles, big stuff and small, purely so that Allied aircraft could spot friend from foe.