It's definitely an issue of training. More to the point? A lack of understanding of what it is they are actually doing - they are testing for small traces of explosives - tiny amounts. THEREFORE? It should not take anyone older than about 10 to realise that IF they are searching for tiny amounts of material THEN the devices used for testing should be kept as clean/sterile as possible.

Given the number of false positives they get - you'd think that would be their first clue.

What is scary is that these geniuses are in charge of our safety.