Yes, it is possible to fly "under the radar".
The type of radio waves that are used by radar and similar equipment to track aircraft generally travel in straight lines. This means that, because of the curvature of the Earth, radars have blind spots at low altitudes when you are sufficiently far from the radar. See the top illustration (not to scale):
People smuggle with private instead of commercial because when boarding a private plane, the security checks are less thorough and very breif. Sometimes, they don't even get checked!
But on commercial jets on whatnot, the security is held to very high standards. That should answer your question.
There are lots of ways to smuggle goods with planes and it depends what you are smuggling and the aircraft on which you are smuggling it. Smugglers often bank on the fact that most countries do not have or allocate the resources to search every aircraft throughly enough from top to bottom before every flight as that is simply impractical on some level. Some ...
pilots are never allowed to have any kind of weapon on board. when pilots go to work they have to go security just like the passengers.
there has been accidents because of this.
its maybe allowed when the total length is smaller then your hand but i`m not sure if this rule ...