The tracking "number" can consist of numbers and letters. You can check it out: just visit http://www.geocaching.com/track/details.aspx?tracker=xxxxxx (insert tracking number instead of x's). If the coin's page comes up, you've got the right number. Then log it there with the logtype "Retrieve from .....".
That was it. Six letters. No number. New coin. It's first location seems to be where I got it from.