This discussion inspired me to look at our donor list and change some donors who moved out a while ago to "Hidden." I took the liberty of "hiding" someone who moved out of the ward this year and also had made a donation this year. Although they are marked "hidden" they still show up on Donation Statement lists, presumably because they donated this year. But, next year, I suspect they will be off the list and that gives me one less task to do in January. Bottom line: Even if they are "hidden" they may appear on the list of donors if they have made a donation during the year. So the mystery of the original post may not have anything to do with the visible/hidden status.
Memories can be tricky. Are you sure the donor actually made a donation in 2010? If so, this is one of those times when it's nice to have hard copies of batch reports in your files. Try to figure out when the donor likely made a donation (which month, for example) in 2010 and search the print-outs to see if you can find their name. Then look for the same report(s) in MLS and try to figure out why they don't match (if, in fact, they don't match). Then check the donation slips that you should also have kept in a file. I'm really curious to find out what happened.
My real name is Greg Anderson.