The reason why certain individuals get pulled over by Customs at the end of their international journey remains slightly mysterious (Border Security notwithstanding), but there is one fairly obvious risk factor: carrying US money. And the reason? It has the highest level of cocaine residue.
A recent study by the University of Massachusetts found that 90% of US bills had traces of cocaine, up from 67% in a similar study two years ago. Other countries' money also shows traces of the illegal drug, but the US ranks the highest -- and its paper notes make it more likely you could run into airport issues.
This I can vouch for from personal experience, after being asked at Melbourne Airport to pull absolutely everything out of my suitcase so it could be scanned. The reason? Apparently there were traces of drugs found on my wallet. It was only after getting the all-clear that the officer explained that US money often has traces of cocaine on it. It's pretty much impossible to travel cashless in the tip-heavy US, but at least now if you get pulled over post-trip you'll know why. Thanks Roulla!
Study finds tiny traces of cocaine on US dollars [Associated Press]