He said that when he called the agent, he was asked if he had been to Boston recently; you can therefore infer that the reason the card was left was so that he would be able to contact said agent(an agent who was looking into him due to the 'photography incident' in Boston)...
It's basically the entire point of the article; although I guess you're right, that aspect is technically implied, not stated explicity. It's a clear implication though in my opinion.
