In early 1896 Tesla had been working with a wide variety of Lenard and Crookes tubes, as well as single-electrode X-ray tubes of his own design, energizing them with upwards of 1,000,000 volts. He speaks in a number of articles about actually melting the glass vacuum envelopes and the electrodes with his prodigious and eponymous coils, which were certainly not available to any other researchers of the day.
This is the earliest reference I am able to find on the use of x-rays for surveilance, covert or otherwise.
From Electrical Review magazine in April of 1896 -Archived here.
“A few weeks ago, when I observed a small screen of barium-platino-cyanide flare up at a great distance from the bulb, I told some friends that it might be possible to observe by the aid of such a screen objects passing through a street. This possibility seems to me much nearer at present than it appeared then. Forty feet is a fair width for a street, and a screen lights up faintly at that distance from a single bulb. I mention this odd idea only as an illustration of how these scientific developments may even affect our morals and customs.
Perhaps we shall shortly get so used to this state of things that nobody will feel the slightest embarrassment while he is conscious that his skeleton and other particulars are being scrutinized by indelicate observers.”
Perhaps not, dear Nikola, perhaps not…