SLK 170 Water in the trunk

Hello, we have at our SLK left and right at the inner side claddings of the trunk water running tracks. And today my friend said, that probably a small puddle has formed. One does not really see the cause, but I wanted to ask if other 170 he drivers also have that. Can not imagine that it comes by opening the tailgate. Thank you already in advance. Andreas P:S.: year 10.2003 SLK 200 K