It looks like an upside-down "3~4" in the bottom right corner. I'd say probably 80% of all $2 bills I get have this mark, from several banks in the Kansas City area and even a few from a bank in Arkansas a while back. Some of them also have "[Presidential Candidate] 2012" written in pen in the bottom left margin.
Common practice for Clerks to write the number of bills they have on the top bill of that denomination.
It is a clue. Keep your eyes out, the rest of the code is hidden on other bills. Once you have all the numbers for the code, it will reveal lat/long coordinates for a discovery of monumental proportions.