Hmm, I tried replying to this thread twice, but it hasn't appeared yet. Here's hoping the third time works.
I'm not 100% sure, but I believe the brake assist radar sensors are behind the 3-pointed star in the center of the grill. If your star has dark plastic infill between the points, then the sensors are behind it.
There are park assist proximity sensors on the front (and rear) of the car at bumper level. Your plate holder may be affecting the park sensors and triggering a warning.
I've ordered a different plate holder (Platypus-Pro from Cravenspeed) that the manufacturer claims is customized to each MB model to avoid interfering with the park sensors (only recent models). It's due to arrive on Monday, and I probably won't have time to install it until the weekend, but I'll report back once it's done and tested.