Hope someone can shed some light...

I keep getting a low coolant light warning. The dlr told me X5's use a lot of coolant and should bring it in to get it topped off. I did and them after a 3,200 mile trip it came on again. Had it topped off again, thought it was the long trip, and 2 weeks later, after normal use, the coolant level warning is back. No leaks that I can see underneath or on the engine.

Any ideas, could the sensor need replacing or should I have the pressure checked for a pin hole leak.