And to skip to what really matters:

According to the Chicago Tribune, a police spokesman said the Grand Crossing police station received an anonymous phone call about the posting. Cops searched Jackson Park and turned up nothing, said the spokesman, but passed on the information to Chicago detectives.

I stopped following postsecret years ago, when it created the forum. It seemed to change the whole idea of it when there was the ability to respond to postcards. You were no longer releasing a secret into nothingness, you were releasing it anonymously into a crowd. This I think made people less likely to share dark secrets (I remember a few animal related posts that had intense backlashes) while also making people want to do it for attention (or well more likely to do it for attention).