Ho ho ho !
No, it's not Christmas, that's me chortling over somebody else admitting that this problem exists.
Although I've never had a customer require that my documentation address this issue.
Over the years I've tracked the percentage of calibrated items that are delivered for re-calibration when due. Regardless of where I worked (granted, only two different outfits for the last 17 years), it ranged from 92% - 97%.
Now, some of that is because I calibrate employee-owned tools if they bring them in - I have a no questions asked policy regarding that. I figure if they can bring it in for calibration, they may also have it in the building and on their desk when a customer or other external auditor walks by, and it hurts less to calibrate those instruments than to deal with the resulting drama of them not being in current calibration status.
And I'm not innocent - I have a couple of meters at home that I commandeered because they were out of calibration, and after resolving that issue they went home with me to work on a motorcycle issue and have not returned.
Employees come and go, and things disappear sometimes. My cal log does include " last known location / user " and that helps me to round up the missing dogies. When an item does not get delivered I remove it from my "active" list and track it as "inactive". Sometimes they resurface after a couple of years, sometimes never.