(This is not historical whatsoever. It did not actually happen.)

Somewhere in the early 1960s, while the Veitnam war was still going on, they developed new combat training techniques. This was when early robots were being created. To train Armies faster in the U.S, the government used a prototype training dummy robot to act as both a target for shooting, as well as being used for hand-to-hand combat. These were later discarded and forgotten forever.

In modern day, you went to the dump to throw away some old stuff. You find a destroyed training dummy, and it powers on. It stares at you.