Low coolant level, but no warning light

[newX3user]

I have a 2004 X3 3.0 . I have read about the previous postings about coolant issues, but I still think I should ask this question. I check my coolant level according to the red stick once a month and I always find it at the minimum level when the engine is cold. I refill it with a 50/50 mixture to bring the stick up to the maximum level. One month later it is again at the minimum level. The warning light does not come on, there is no smell in the cabin, and no leaks under the car. Actually there is some leak under the car, but it looks like water. Anyway, is it normal for these cars to lose some liquid during one month?. (from maximum to minimum)