Column density of the CSM shells as a function of the spectrum’s S/N. Triangular markers denote the lower limits for objects with no detected shell, representing the column density above which a shell would have been detectable given the noise level. The dashed line shows the best-fit relation, and the shaded region marks the parameter space where a CSM shell would remain undetectable at the given S/N. Star symbols indicate the measured column densities (or lower limits) for objects with detected CSM shells.