[ionicgiraffe]

Not still a good analogy, but radiation can be seen as a consequence of vacuum not being empty from the perspective of an electric field (it has a very low permittivity, but it is not zero, so it is not empty of that property) even though it is empty of mass. If your water pipes are a bit porous and the medium on which your water pipes are was not air, but a very dense fluid, radiation emanating from the porous pipes would travel at the speed of sound through the medium, and could be detected without particles of water actually travelling through the dense fluid that constitutes the medium where your hydraulic circuit is submerged.