WARNING: This page is about a different car, the 2013 BMW 550i GT, 2012 BMW 550i GT, 2011 BMW 550i GT, and 2010 BMW 550i GT. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Identifying C-Pillar Reinforcement, Cavity Acoustic Baffle, Severance Cut Mark And Welded Connection Areas
Courtesy of BMW OF NORTH AMERICA, INC.
Mark severance cut (1) next to C-pillar reinforcement (2) and cut