Odd occurrence or security issue!

Odd thing happened while entering an address in Outlook, I typed the name of the person and Outlook completed the address! How the heck did Outlook know what the tail end was?? What I finally surmised is that I have received mail but "ONLY" as blind copies with this address, I have never received or sent mail to this person, that leads me to believe that Outlook stores all incoming addresses even if they are blind copies. This seems like a privacy issue to me, if OL stores addresses then they can possibly be retrieved by an outside source.

Phil