I had this problem 3 weeks ago and took my X5 to the dealership twice for them to reset the warning light because the tire pressure on all 4's were correct. However they cautioned me that there might be a very slow leak in one of the them. Sure enough, on the third week, the light came back and I saw that the driver's side rear tire is a little deflated compared to the other 3. Took it to a tire shop and they pulled out a nasty nail.

Give it a few more days and check your tire pressure again.