Speculation; but such a story is worthy of speculation. They probably have not been 'found' because they possibly were never 'lost'. They 'may' have been runway/airfield denial explosives now perceived as timex and therefore in need of removal or replacement. If it was of WWII legacy, they would have said so, and given the security on US facilities being somewhat more robust then typical UK bases, I cannot for one minute believe that explosives are from an unknown source.
The lack of specific information will of course give rise to decades of conspiracy theories amongst the local community. It is not the best example for community
PR I have seen.