This cap sits on the peddle end of the Bee and Bee Plus brake rod. It acts as the cam follower to the Peddles cam action. If your brake rods are not coming out far enough to engage this is the problem. Cap fits either side and a 3 mm drill and pop rivet set is needed to fit this.
Another repairer in Norway has a good blog with photos about this problem http://barnevognverksted.blogspot.co.nz/2015/10/bugaboo-bee-plus-defekt-bremsepedal.html?spref=fb