I have had an S205 (the estate) since December, and for the first time I think the boot may have opened itself. It's happened overnight while at work. Only possibilities I can think of are:

I've accidentally opened it while walking away from the car, which is unlikely because the keys were in a loose pocket, and if it did open I would have heard it.

I work at a place where the is RF interference. We don't try locking our cars because if they do, they might not open. I wonder if that has somehow opened it.

Or maybe it's just a fault and it opened itself.

 

I had this happen to me as well a couple of weeks ago. Went out to the garage and the trunk was wide open. I just kind of chalked it up to the fact that I must have bumped the trunk button on the FOB in my pocket as I was running around the house gathering a few things up before walking out the door. Hasn't happened again since that one time though.

 

This happens to me constantly, because the FOB trunk button is pressed in the pocket of my jeans (while picking something up, or sitting down) ... The W205 reacts too quickly to the trunk button: much quicker than e.g the W203 with the same (new) key did ...

Very annoying when neighbors have to ring my doorbell (again) to tell me that my trunk is open in the snow/rain ...

 

I'll be buying a key cover...