Errata Security releases code for program that reveals what your Wi-Fi connection is "leaking": David Maynor and Robert Graham say that your Wi-Fi-enabled computer "seeps" all kinds of data relevant to crackers regardless. They have a great definition of this term: "Information that is broadcast or available via simple inquiry or spoofing that may not by itself seem critical but become more important as pieces of a larger puzzle."
Wi-Fi networks are particularly troublesome, because some operating systems and drivers look for known networks that they have connected to previously; spoofing one of those networks grabs you lots of this seeped data without compromising the computer in any fashion. Given that a list of network names might include airports and hotels, or the BSSIDs associated with them (which could be gathered from wardriving information already out there), you might know where someone has been.
You can download their presentation on the matter and the 1.0 release of their buggy--their term!--code.