I assume you're referring to: Inspection, Measuring, and Test Equipment.
You might consider assessing your failures 2 different ways: 1) instruments that exceed specified tolerance; and 2) instruments broken or lost.
For 2) once an instrument is reported broken/unusable, I would attempt to repair/send off for repair/calibration. Typically the repair service can assess the initial condition. It could simply be a blown fuse, and was totally detectable by the user. These types of activities where that instrument failure (up until the time it stopped being used) had no impact, I would state a failure notice wouldn't be needed. The work performed and the non-scheduled calibration can be documented on a work order/ work request.
If the instrument malfunction is not detectable by user, or could have potentially impact the readings/ process, a failure notification should be issued. On the failure, the end user can document if the failure was discovered during use, if there were any studies/measures that appear to be affected, etc. This is a "best effort" documentation.

After all, the instrument was lost or just stopped working. But at least trying to assess if there appeared any process impacts and such is better than nothing.