@australian...deice... Was not There, but dont think so, mild weather at Schiphol.
In a relatively recent case they had deicing fluid in apu space. Cleaned it and other spaces up thoroughly and effects disappeared. Only to return and disappear again later in specifically that aircraft. Two FA ended up with serious permanent issues, many others pax and crew no issues at all. Operator decided later to stop deicing with apu running after they found that glycol values went over the limit during tests. Just to be sure.
Investigators and airline tested the ac on multiple occasions on dozens of chemicals with multiple sniffer types. All values significantly below allowable values. Crew tested too.
Problem with the chemical approach could be that limits are based on single substance tests, so not on combinations. For that you would have to devise a more advanced BD-test.
But before doing that i would suggest taking a look at a number of other potential causes. Causes not directly related to 'chemical' that is.