Modern encrypted spread-spectrum digital radio would have looked like random noise until quite recently. I'm not sure if you'd pointed all the technology and all the brilliant minds that existed in 1930, at a modern cellular tower, that they'd have recognized the signals to be coming from a technological civilization.

In another century we'll no doubt be hugging the noise floor even more closely. It's not hard to imagine that even if alien civilizations are broadcasting omnidirectionally all around us, that we're simply too primitive to bother targeting with a message. Why waste the effort when you'd be able to communicate so much more information, and more efficiently, by targeting just a slightly higher baseline of receiving technology?