Learned this one second hand from a smuggler:People tend to notice things they're looking for and once they don't find it (or even better, they do find it) they ignore everything else. So if you want to slip something by someone, make sure they catch you on something that doesn't matter, and they wont focus on what you're hiding.

The story behind this is that in Berlin while the wall was up, there was this smuggler who used to ride, every day, from east Berlin into West Berlin (or perhaps it was vice versa, I don't remember) with a bag of sand on the back of his bicycle.

Every time, the checkpoint guards would cut open the bag of sand, search through it, find nothing, and let him go on his way. And every night he would return.

They never realized that he was smuggling bicycles.