The ones I've seen are usually green, but I've also seen at least one red one. They are numbered. The highest number I've seen is 54 which seems to indicate there are quite a few of them out there.
There's one on a park bench in Lawrenceville on Butler (near the old pool). There's another on a utility box at Stanton and Morningside. I found a third today in the Strip District on a mailbox between 21st and 24th on Penn. I know I've seen a couple more, I just can't recall where.
Anyone have any ideas what this is about? I considered some sort of geocaching thing, but they're not well hidden (for example, the one in Lawrencille is on the front of the bench, not the back. Anyone sitting on the bench, or just walking by, can see it).