Given the low information bandwidth -- single-digit bits per second, transmitted in bursts with an extremely low duty cycle -- it's impossible to detect such signals in the general case. One way to conceal conventionally-modulated pulse signals would be to superimpose them against legitimate signals like local FM broadcast stations, for instance. Another strategy would be to use spreading codes to randomize the timing of very brief pulses, rendering them completely indistinguishable from static.
The best way to catch this type of activity would be by detecting the presence of the equipment itself, such as with a nonlinear junction detector. This is a piece of gear that generates a small number of strong, spectrally-pure RF signals and listens for distortion products that indicate the presence of semiconductors nearby that are acting as mixers (which they will, whether they're designed to or not.) You can find anything from a hidden bug to a rusty rivet on a gutter that way. Even so, an NLJ sweep is no guarantee of success. The active devices in the transmitter and receiver could be biased into hard conduction or cutoff except when being used, for instance.
People have joked about "anal beads," but the fact is that a buttplug would be a near-flawless platform for such a communication system. You have the fine muscle control needed to transmit your opponent's moves to an accomplice via any number of pressure or motion transducers, and obviously you have the ability to perceive electrical or mechanical impulses received in return. If X-rays are ruled out for safety and if cavity searches are ruled out for modesty, detecting something like this would be a challenge for the highest-level government TSCM teams.