You will not be able to see a crack initiation site without removing the spring and carefully examining the fracture surfaces. Do not clean or disturb the rust or coating in case Bilstein wants you to return the spring for analysis.

The topmost coil is ground and terminates in a taper. It's called a square end as shown here Types of Coil Spring Ends - Duer Carolina Coil, Inc. The end taper contacts the next coil wrap, which may result in loss of protective coating and corrosion. The photos show corrosion where the adjacent wraps contact. Note the loss of coating a little ways beyond the taper and above the fracture. This indicates long term coating failure due to corrosion spreading under the coating.

You don't have to abuse a spring to produce this type of failure. Once the crack initiates, it grows each time the spring is flexed, forming characteristic striations, visible under a microscope after the fracture surfaces have been properly prepared for examination. The striations visibly indicate where the crack initiated and once identified will likely determine if the cause of failure was corrosion pitting, a material defect, a point of stress concentration, a manufacturing defect, or a design deficiency.