Early reports over the weekend like this from the BBC said [The question of no warning is harder to answer. Volcanologists point out this was a relatively small eruption, and that it was driven by super-heated steam and ash, not by lava being ejected from the magma chamber. That made it much more difficult to predict.]
But now the ABC is reporting – [While there had been a rising number of small earthquakes detected at Mt Ontake since September, the eruption could not have been predicted easily, according to the Japan Meteorological Agency.]
I have looked for Japanese seismic data for Mt Ontake to no avail yet. I also wonder if they have networks of surveyed points over volcanos to detect subtle changes in topography which can precede an eruption.
Active volcanos are individually monitored for seismic activity, gas effusions and ground movements (uplift and subsidence).
This page holds the monthly reports of volcanic activity.
For example, April 2014 (the latest report) has:
And: