Concealment of signals
Gee, I happened to look at Wikipedia's article on information theory and, under applications, found that concealment of a signal under the noise floor is used by GPS. Additionally, one uncovers the signal using a secret pseudorandom function.
So, though I didn't check my idea (below) with an expert, it appears that I was on the right track. Again, I note that the presence of a pseudorandom function may be detected with statistical analysis.
Also the Wiki piece did not specify that the GPS system's pseudorandom function aimed to make noise power roughly equal to signal power, thus making the detectable signal about zero.