All sorts of things, probably including insects, could potentially cause False Positives.
Depending on the previous filter replacement cycle (ie. when clogged with non-ash material and therefore doing their job), you could either set a maximum for ALL air contamination permitted on one flight and quarantine the aircraft whenever it was exceeded, or find that for filters tested after one flight only, the level of contamination from non-ash causes will always be insignificant. Makes no impact on the effectiveness of the filter testing as a first-line defence against ash in the turbine, I don't think.
Might be a good idea to monitor ALL the different crud passing through the turbines, anyway!
On further reflection, my suggestion seems so obvious (and already widely done in other contexts) that it's hard to understand why it's not been done with volcanic ash or other intake air contaminants since forever! Next thing, some engineer from BA Cardiff will pop up and say they've actually been doing it all the time (or at least since the WW
PR flight!
