This script returns a report (a temp TXT file) about a swatch usage.

If you’re running into a difficult to remove a swatch (and don’t know why InDesign doesn’t let you to remove it), this script could be useful. Since swatches used as style properties (paragraph shadow, rule, etc), even if the property is turned off, InDesign won’t let you to delete the swatch. Knowing where it’s being used, you can remove it without doubts.