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.
The following illustration shows the arrangement of the power steering components and lines.
Fig 1: Overview Of Power Steering Components
Courtesy of BMW OF NORTH AMERICA, INC.
Check all visible connections, hoses, lines and steering gear for traces of fluid.