Judges 3:15-23 tells the story of Ehud, who freed Israel from the Moabites by killing their king. The story tells of the assassination in detail. Ehud was left-handed, and concealed an eighteen-inch blade on his right thigh. He then asked told the king that he had a 'secret message' and the king sent his bodyguards away. Ehud then stabbed the king and made his escape.
It never explicitly said that they looked for weapons on his left hip, where most people keep a sword, but the assumption is that they would. This story is often used as an example of how people can accomplish things because they are different. "The guards didn't see a wepon on his left hip, and never imagined that someone would conceal a weapon somewhere else, so they trusted him alone with the king. Yay lefties."
I know that left-handed people were much rarer in antiquity than they are today, but the behavior of the king and his guards is still idiotic according to that theory. It doesn't take very much training to learn how to use a blade in your off-hand. Any right-handed person capable of using a sword could pull off this stunt after practicing for a few hours at most.
Even if you were completely incapable of using your left hand, there are at least two ways to draw an eighteen-inch blade from your right hip with your right hand. You can pull it out so the blade is pointing to the ground, and stab downwards. Or you can twist your hand upside down when you draw it, and then flip it forward so you are holding the blade normally.
I know they didn't have any kind of advanced combat training in biblical Judea, but surely the budyguards of a successful king would have known about these kind of basic blade stunts. I'm guessing that the real reason Ehud got in was because of attitude and psychological manipulation. It is clear from the story that he was good at deception. He probably just didn't look like a threat, so they didn't bother to look closely.
That hasn't changed in the last three thousand years. Good cops and securuty guards read people by their attitude. It is pracically impossible to scan a crowd for concealed weapons, but law enforcement professionals can easily spot the person who looks tense, nervous, or excited. If you keep your cool, they don't bother you.