The man's name is John Hodgman. He has written a little tome that I'm sure you're aware of by now: The Areas of My Expertise. It is a full-on almanac of everything. If you haven't bought your copy yet, you're foolish. If you bought your copy in hardback, you're foolish also because the paperback has even more of everything in it. Consider him your living, breathing librarian at Alexandria, and the book is your library card. Enough. Buy it already.
However, there is more. One of Hodgman's many areas of expertise is hobo affairs. And no, I don't mean where Snuffly Pete is sneaking around behind the boxcar with Old Jane Knackenbush without Patch McPhlegm being any the wiser. I'm talking about their activities. Specifically, their symbols. He wants any evidence of hobo signs reported to this Flickr pool.
It's best not to ask for details. Some things we're simply not meant to know. Until the sequel to Areas comes out, naturally. Just do that thing.
And once you've done that, head to Quick Stop and check out this interview with him so you too can learn more.