Just wondered. I have been a member of the WheresGeorge.com site for several years. I have spent many of my marked dollar bills at Kroger, as well as many other places. 

For anyone not familiar with http://www.WheresGeorge.com,  basically, its a fun (and totally legal) website which has been going on since 1998.   There, anyone can "enter' any bill (1, 2, 5, 10, 20, 50, or 100 dollar bill) into their database (typing the serial number & year series) then write or stamp the bill with the words "WhereseGeorge.com" placed on it somewhere. (There are websites that sell custom-made WG stamps to use). Most users stamp the bill in red ink.

Then, you just spend your marked bills anywhere, and sit back and wait........   for someone, somewhere, sometime, to get curious enough (if they notice the markings) to go on the http://www.wheresgeorge.com site and type in the serial number on the bill.  They can also leave an optional note about where they found the bill.    When they do, you get an automatic email telling you what town/state where the bill was found (the zip code to be more exact). 

It is amazing how money travels.  Bills circulate all around the country, and sometimes show up in foreign countries.   

Many of the bills are never "heard from again",  but,  on average about 15 to 20 percent will eventually get a "hit".  A "hit" is when someone re-enters your bill at the site and you get a notification.