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):
What you propose is called a monopulse radar. Monopulse antannas have two feeds and beams, slighly to the left and right of boresight or above and below. In transmit mode, both feeds are connected in parallel to create centered beam and a single pulse is transmitted this way. In receive mode, the phase or amplitude difference in the two channels is used to ...
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 ...
Online calculator for the same: