Saki's parents leave her a letter describing, roughly translated:
Most of the weapons non-cantus-users created to defeat the cantus-users have been rendered useless throughout the years. However, one of them managed to survive this whole time: the Psychobuster. Here is the address of its possible location - if you manage to find it, you might be able to stop the demon.
The letter implies that they know (or at least, are fairly sure) that there is some Psychobuster out there that survived those centuries.
Of course, they aren't sure if Saki will be able to find the Psychobuster in the mentioned location, but they ARE sure that there must be one existing somewhere, which is what concerns me. How can they tell that there is still some left?