X-Ray lens technology was investigated way back during WW2 and then abandoned: it didn't work well at all because the moment the target was found, it became invisible because the lens allowed the operator to see right through it - the target, that is, not the lens, though in a sense, you could do that as well, if you were the operator.
That's why Kilroy was brought into play: his alternative methods of snooping became both highly efficient and, ultimately, legendary.
Rob C