Turns out that the coworker who complained was a transwoman. The complaining part is sadly not a surprise, since we already know that some transwomen think women are being female at them out of spite, but the fact that HR actually took the complaint seriously and wrote the letter writer up for having personal care items in a bag in her car rather than explaining to the complainer that the complaint was unreasonable is not exactly a great sign in terms of how workplaces are handling complaints relating to trans issues.
www.askamanager.org/2018/04/i-got-in-trouble-because-my-coworker-saw-maxi-pads-in-my-car-and-more.html