Although it is possible that TEPCO is hiding that one, but as far as the official news goes which NHK reports dutifully, there is no 300-milli-sievert/hr debris on the mountainside but that's what NHK WORLD reported already, both in news video and on the article on their website.
As if a highly radioactive debris flew from the Reactor 3 and landed on the mountainside. I've found some English sites already quoting NHK as saying exactly that.
The original Japanese version of the news only talks about the debris located near the Reactor 3 reactor building, on the side that faces mountains - i.e. west. There's also an "ocean-side", the side that faces the ocean - i.e. east.
I guess the NHK translator doesn't know the plant orientation yet, and neither do the editors. Thus, NHK World inadvertently joined foreign news organizations spreading "false and malicious rumors"..
From NHK World (4/24/2011) "TEPCO discloses radiation map":
... Radiation levels around the Number 3 reactor building, which was damaged by a powerful hydrogen explosion, are higher than in other locations, and 300 millisieverts per hour of radiation was detected in debris on a nearby mountainside.
In the accompanying video, the announcer reads exactly the same text with absolutely no hesitation or doubt.
And this is the original Japanese news, from NHK Japanese:
(My translation) Radiation level is higher around the Reactor 3's reactor building that suffered a powerful hydrogen explosion. From debris found on the side of the Reactor 3 building facing the mountain, 300 milli-sievert/hour radiation was detected.
I have sent an email to NHK WORLD warning them of the translation mistake. Maybe I should call them and warn that they are spreading a "false rumor" to the entire English-speaking world...