This should hardly come as a surprise. After all, didn't a box of inventions created by Superman's evil cousin, the black sheep of the El family, Kru-El, once reach earth and was discovered by Lois Lane?
There have been various occasions where Kryptonians have interacted with earth and thus could have left scientific objects behind, such as during "The War Between Krypton and Earth" in the distant past of the Legion stories, and further, Circe the Sorceress in Ancient Greece was Kryptonian and had access to their superior science, which she used to duplicate magic.
On one occasion (whose issue number I forget; does anyone have it?) Lex Luthor discovers a cache of Jor-El's inventions, including a "puzzle ray", which good old Lex wastes no time at all in using against Superman.
One very cryptic hint was present in the first appearance of the Cosmic Anti-Superman Gang in an issue of SUPERMAN'S PAL, JIMMY OLSEN, seen here:
http://img126.imageshack.us/my.php?image=jimmyolsen08705sm6zj.jpgSuperman, cravenly pleading for his own life, promises Luthor a variety of Kryptonian hidden Super-Weapons. At the end of the issue it turns out that Superman's craven behavior is in fact a charade, however, to tell Luthor that he would reveal to him where caches of Kryptonian super-weapons hidden on Earth are in exchange for a pardon...is a very bizarre bluff to make, to say the least.
Does this mean there are in fact, caches of hidden Kryptonian weapons on earth? Hopefully, it might be interesting for future writers to deliver on the promise that this story made.