some other form of communication that we are unable to detect.
If there is such a civilization, let's assume that they have figured out that form of communication. All we have to do is to stick some rocks, bits of wire and some twigs together to find it. We might not be able to detect it efficiently enough or correctly demodulate the signal, but just finding the presence of a signal would be a breakthrough.
Think back, it would have been possible to build an AM receiver a few thousand years ago. All you need is a coil of copper wire connected to an interleaved pile of gold foil and parchment. That's a tuned circuit, now hook that up to a rock (galena) and you have most of a crystal radio. A piece of quartz with some foil stuck to it might be enough to act as a sound transducer.
What would it have received? Probably just static from a nearby thunderstorm. Once someone worked out that lightning made the quartz crackle, it could have served to jump-start the development of electronics. That development may have been totally mis-guided and cloaked in alchemical style jargon, but it would have been progress where there was none.