"Around 2400 finders reported their finds to the Scheme"

No mention of whether they were all detectorists. I've assumed above that they are all detectorists which makes the non-recorders look (marginally) less like brazen thieves.

If we assume that is actually all recorders and that 68% of them are detectorists (given 68% of the finds are from detectorists), then the figures get scarier.

So lets play Ballpark Mathematics.
Assumptions:

- 14000 active detectorists
- 2400*0.68 = 1632 responsible detectorists
- 30,000 * 0.68 = 20400 finds reported by detectorists.

And so
(1632 / 14000) * 100 = 11.65% of detectorists are responsible

And (14000 / 1632) * 20400 = 175,000 finds found last year that weren't reported by detectorists.