The article said he didn't. He was just taking a quick smoke break just outside the building, so he only planned on being gone a matter of minutes.

I've visited these large NYC office buildings on weekends on many occasions. They're deserted. Sure, there are some people around but they're totally scattered throughout the building. And even if someone heard the alarm when they were taking the elevator themself, I have to admit that it would be very easy to assume that the building management had it covered and just not say anything. This story reminds me that we shouldn't assume that, even in a big, fancy office building with 24-hour security in the lobby.