Something like 85% of currency in circulation tests positive for at least one controlled substance when random samples are run by the Fed. That's what happens when things come into contact with each other.

Same thing for your socks. If you removed your shoes to walk through the checkpoint, you walked across a space which thousands of people walk across. You also touch other objects all day long. "Trace" means "trace" and it means that you were in the Chicago area and that there is cocaine to be found in that area. That isn't news.