2hats
Dust.
It's difficult to eliminate confounders. It was thought sensitivity had dropped somewhat in the era of omicron, but in the end the hypothesis was that this was actually a consequence of vaccine (and post-vaccination infection) driven immunity producing earlier symptoms, as the trained immune system more rapidly kicked in. This tended to lead to testing sooner in the infection cycle than in previous waves, but this was before there were sufficiently high nasopharyngeal viral/antigen levels to reliably register a positive result (would perhaps take another 2-4 days along in the infection cycle compared to earlier in the pandemic).Wasn't there a variant that wasn't picked up on the tests as much as others?