2006 Mercedes Benz E320 Leak?

Hello guys,
Since I never drive behind myself I have not noticed this type of problem before so not sure if it has been ongoing or if it just started today... But this morning on the way to taking the kids to school, a man stopped me and politely told me that somethng was leaking from my undercarriage. I bent down to look under the car and sure enough, there was a steady stream - not gushing - but streaming steadily... the stream about the width of a 1/2" - 1/4" ... from the very back end of my estate. When I tried to really get on the ground and get under my car (very difficult to do as it is a Sports model and very low to the ground!) I could see that the area it was coming from was the large, enclosed "black box" area at the very back end (I hope this makes sense). It had stopped doing it by the time I got home and the fluid seems to be water ... Not oily like other fluids. No lights have come on to indicate low fluids of any sort. Is it possible that this is just the same thing I see when I am say driving behind a bus and it has what appears to be water coming from the back end in a steady stream? In other words, just condensed water, or excess collected water from the past wet days over the weekend? or something? I hadn't driven my car in two days and it has been raining! I don't know and now I am worried but definitely would NOT want to take it to the dealer because - as luck so often has it - my warranty has just EXPIRED. Yikes! I am ever so grateful for any light you can shed on this matter. Keep up the great work! Best, Anne Marie