As with any particles in clear air whether moisture or volcanic ash they are visible in day light in clear air in the form of clouds or mist.
Ash layers are not always visible in daylight, it's possible for them to be completely invisible - as happened in some areas after the 2010 eruption.
The last time was a ridiculous fiasco with computer generated prediction flows which were totally inaccurate and for what?
As far as I remember the modelled ash dispersal was actually reasonably accurate. The concentrations were not.
There has never been a fatality in the history of aviation although there have been close calls flying into thick dense ash easily visible to the eye in daylight while flying in clear air.
There's never been a fatality associated with flying a widebody aircraft inverted under the Sydney Harbour Bridge either...but that doesn't mean we should try it.
'This has never happened' is a terrible argument to use, just because something has not happened thus far is no guarantee that it will not happen in the future.