In the course of "exchanging" items its quite likely you'd be emailing concerning the trade. If your "exchangor" was working for the airline all they had to do was compare your email address to the email address on file for your account and bingo, they know who you are. In the event of a non-sanctioned trade (swapping miles for upgrades, selling upgrades, etc.) they'd audit and shut down your account (not necessarily in that order).