Consider this:

To date, have you observed any other countries and governments undertesting? And if so, was it for reasons other than the Olympics?

That gives you a baseline of disincentives to report.

Now extend: - what would the cost (financial, reputational, etc) be to Japan the government if the Olympics do not go ahead? - might there be other disincentives for Japan to undertest relative to its neighbours?

Also recall, in some cases a little bit of disincentive mixed with poor planning/incompetence leads to a lot of unplanned underreporting :)

The trick in all cases is to look for a metric that governments have not thought to obscure/or cannot control that strongly signal the thing you’re trying to verify.