Why would a hijacker abandon weapons? If their goal was hijacking and they found they were to be searched then why even board the plane? Just go back home with your weapon.
I think you might be overestimating the amount of planning that went into 1970s hijackings. One person hijacked a plane with nothing more than a can of bug spray and a lighter. Another was an ex-Green Beret who came to believe that he was at the center of a CIA plot to Assassinate Castro alone. For the most part, hijackings in the 1970s are best described as "clown shoes shit".
Hijacking was different in those days. It was often a means of political protest or transportation.
Another thing to consider is non-hijackers carrying guns for their own reasons. It was fairly common for business travelers to carry a lot of cash, and carry a gun while doing so.
> Hijacking was different in those days. It was often a means of political protest or transportation.
Indeed, that was one of the major shocks (and changes brought by) 9/11: before that in the case of highjacking you wanted to remain calm because you were at relatively little risk of harm, in the early 70s highjacking were a weekly occurrence and treated as inconvenience. Jacking gradually went down through the 80s as security screenings were added.
The danger was in bombings and there was not much you could do as a passenger.