I agree with Yal that the leak is at your water pump. I was losing coolant and couldn't find any on my garage floor. It was leaking out when the car was running, and would evaporate before it would leak on the garage floor.
On my last car (a Nissan) I got two bad water pumps in a row from the dealer that broke within a year. The third pump was the good one.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".