Suppose you want to bug a room and you know somebody's going to sweep it. It's well known that pretty much all radio frequency emitters can be detected using a careful RF analysis. Basically, if it's emitting energy, it doesn't much matter under real scrutiny how you try to divide that energy. It's radiant energy, and as such it can be intercepted and detected.
Unless, that is, it can't be told apart from what the sweeper is expecting already, or alternatively is so utterly different from what se's expecting that se doesn't even have the equipment to efficiently detect it.
From there, I can come up with two examples already. First, powerline hum. If all you put out is something very close to the extremely strong hum always present in any modern building, you're not going to get caught. And even if people start to pay attention to this sort of thing, you can always lower your transmission rate, driving the effort of detection upwards by as much.
And second, since that leads to a diminishing channel, you can play the age old game of finding new modes of conveying the information out. One such means that I haven't seen detected yet is coherent polarization modulation. Say, you place your transmission right on top of some prominent microwave emission line of a compound that is widely present and put out a low level, constant envelope additive signal. Only your signal encodes your stuff via varying polarization, not amplitude of frequency. Just try and find the equipment which can deal with this sort of stuff at low levels...