"He gives one [gift card] to each of his assets and tells them, 'If you need to see me, buy a coffee.' Then he checks the card numbers on a cybercafé computer each day, and if the balance on one is depleted, he knows he's got a meeting. ...

Saves him having to drive past a whole slew of different physical signal sites each day [to check for chalk marks and lowered window blinds]. And the card numbers aren't tied to identities, so the whole thing is pretty secure."