Your thermostat is faulty and is sticking in an open position allowing coolant to keep circulating when it should close up and stop the flow.

This is how it keeps the engine temperature fairly constant and means that the needle never moves once the car is up to temp.

I've had this on 3 previous BMW's in the past.

If your engine never stays at the optimum temp then you will run slightly rich and this will affect fuel economy. Change it now - if you're going to do it anyway why wait. Its not a massive difference but its not optimum for the engine either.