2005 X5 3.0i sport

A couple months ago, I had the thermostat and coolant sensor replaced w/ coolant flush/fill. I just randomly checked the coolant level and it was low so I re-filled it. It went back down again. I don't see any wet spot on the floor of the garage. Before I take it to the shop, where should I look for any clues?

Note: Low coolant level warning never came on.