Yes, this happenned to my 1994 C180 Euro last week and I saw this patch of water at exactly the same spot as you mentioned around the black rubber grommet. However, it wasn't so serious as to drop on the floor under the car. I pressed the reservoir at different angles, but couldn't induce further leaks after drying up the spot. How it happenned I cannot guess. If there is a leak, pressure around the bottle should cause water to escape from wherever the hole has appeared. Now it remains dry and I am not bothered. Why is the grommet there? Is it an escape hole or is it holding on to something??? I heard that the bottle always leaks and this is a sore QC problem with Benz. Has never happenned to any of my Jap cars for the last 30 years!!! Amazing.