I was referring specifically to checks for volcanic ash.
Although Mark 1 Eyeball checks on pitot heads etc. and accessible surfaces where ash MIGHT collect are obviously part of what is done anyway, there is plenty of evidence (quoted here and nearby ad nauseam) that negative external visual checks do NOT necessarily correspond to what's actually happened to the inside of engines, in particular the turbine hot section. Please correct me if I'm wrong, but borescoping at least one engine on EVERY potentially-affected aircraft whilst parked at the ramp is not a viable option.
And what about cumulative data? Obviously, the same aircraft will fly multiple routes during a Volcanic Ash alert. OK - so it passes the visual checks every time with flying colours. But there MAY have been multiple undetected ash encounters, each one contributing to the total accumulation sitting in the turbine cooling system
And what about all the OTHER aircraft from other operators that will fly the same route?
If, as an expert in the area of ash checking and engine maintenance planning, you're totally happy with the existing checks and tests, fair enough.