Zero-result searches
Measure not only how many searches return no results, but why.
A declining no-result rate is only positive if visitors get better results.
Classify causes
Separate missing assortment, data, spelling, customer language, filters and index problems.
Weigh volume
Prioritize recurring problem terms.
View follow-up action
Measure reformulation, exit and alternative product click.
Prevent fake results
A broad fallback that shows irrelevant products should not artificially improve the metric.