Since you topped off the coolant and haven't lost coolant over the last 1000 miles, I would just check it frequently and also take a flashlight. Now if you are going to be driving long distances you should keep looking. Suggest you check heater hose connections and the water pump. Those type of leaks can be very small in the early failure stages.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas