Everyone seems to know one card trick or another. This one can be done with almost anything. The reason cards are often used is their inconspicuous nature and low cost. Consider this, whenever you or one of your neighbors go on vacation does anyone look out for the property? Collect newspapers, mail and clean up anything on the property that you might clean up anyways? This ties back into the concept of "covert channel" communications. The lack of presence at a property could be discerned by the lack of routine activity. Throughout our lives we set patterns and our individual patterns blend with the various groups around us. Someone paying attention to these patterns can easily see a change. A car left parked outside all day and never moved. This might be the second family car and the lack of movement could tip off a bad guy that the family has not been home. A dog that is normally outside in a fenced yard that is conspicuously missing during otherwise fair weather. Any of these could be a tip to the bad guy casing the neighborhood. And by casing, or observing, they could be exposing themselves to undue scrutiny by others in the neighborhood. To avoid this scrutiny the simple card-trick method is used. In addition to avoiding undue scrutiny it also permits the screening of a large number of target residences in one day - without the activity attracting too much attention by itself.
How many times have you come home to find a business card or flyer at your front door? Did you follow up with the business to purchase services? Probably not. Now if a bad guy, or team, were to blanket a neighborhood, or several neighborhoods, with these solicitations it might take a couple of hours. Say half a day tops. The next day, or maybe two days later, they might drive through the same areas and identify those houses that still have these items on the door. Would your neighbor know to check your front door and collect these items as well as mail and trash? In this way the bad guy can identify a number of targets with little effort. No doubt, at least one home owner might catch them at the door depositing the card and make an inquiry. The screening bad guy need only say he/she was just paid to deliver the cards and thereby avoid further inquiry. Or a more elaborate script may be followed whatever works best for them.
That's the card trick. Some sort of debris is left at a residence to identify a lack of attendance to its care. It is fairly easy to defeat this approach through diligence and neighborliness.