...Somehow I don't think that's the case here, more of the mages going 'okay, we can't let anybody know about this Grail War we have every once in a while because it's secret and we're secret, where do we put it?' And then they stuck it in a cave that they magically covered the entrance to. And then in the third Grail War Angra Mainyu corrupted it, so Issei's basically been living above an evil-spewing abomination all this time.

I'm going to guess that that doesn't happen in real Japan.