"Energy signature"

(This is a non-political weekend post) The plot of the not-altogether-successful-but-better-than-I-expected-although-it's-surprisingly-gory-and-I-prefer-the-expanded-DVD-cut Watchmen movie hinges on a never-defined term: "Energy signature." At two points in the story, the United States government locates Dr. Manhattan by recognizing his "energy signature" -- a phrase which, if I recall correctly, does not appear in the original comic.

The term also shows up in other science fiction-y stories. It has become a trope.

So what is an energy signature?

I'm 99 percent convinced that this phrase has no real meaning -- it's just a literary device used by somewhat lazy writers to make the plot work. (Sort of like "applied phlebotinum.") On the other hand, there's a lot about physics that I do not know and never will know. So let's throw the question open to the readers.

"Energy signature." Is it, like, a thing? Is it real? Does the term appear in any textbook, and if so, can anyone scare up a definition?

Question 2: The end of the Captain America movie. Couldn't Cap have at least tried to land the big Horten-style plane on a remote runway in Canada? The plane, incidentally, had computer screen read-outs, like a modern jet. If the Red Skull had such advanced tech, how did the Nazis manage to lose the war?