>>Now I have a picture of Jon dropping off a Christmas card at the
>>*real* source PO box every year. Dan knows Jon's going to be there,
>>so he clears out that day. Jon takes some comfort in the fact that
>>the PO box isn't accumulating mail, so somebody's cleaning it out.
>>
>>And they're both OK with the status quo.
>
>"You sent Dan a Christmas card with a tracking device?"
>"Yes. And every year, he sends it back. If he ever fails to send it
>back, I'll know there's a problem."

And that's even better! Brilliant.