With any big race, never mind the Boston Marathon, there's a huge crowd at the finish line by the time most of the runners have crossed it. Families, runners who finished and want to sign up to get a still shot of themselves crossing the line, press, general spectators. Security is there, but relaxed compared to the start of the race or the elites finishing. It's a smart time to slip through and drop a couple of duffel bags.
You know, if that's the sort of thing that gets you off.