Despite rumors to the contrary, the police aren’t stupid. They are trained to ask questions in ways that elicit a confession or falsehood.

The simplest example is asking “Do you know why I pulled you over?”. Typically, people spontaneously confess to speeding, sometimes they break down and admit that someone is wrapped up in a rug in the trunk.

The courts have consistently ruled that customs is different and you can be searched without a warrant. Don’t cross borders with contraband or evidence of criminal acts/dissident identity/your email correspondence with foreign agents/etc.