I've received two $1 bills recently with markings on it. They had a web address on it: Where'sGeorge.com? You enter in the serial #, denomination, series and zip code then it shows you the path this bill has taken. It looks like bills are entered fairly infrequently, I guess nobody pays much attention to their money.
The 1st bill arrived on June 12th. I know this now thanks to my browser history.
The 2nd bill arrived today, I found it in the till when I came in to work.