This is a problem; but we also don't know what sensing technologies are available in the future.

There is a very interesting sensor that I was reading about; it uses entangled photons (microwave) one is sent to what is to be detected in a electromagnetically noisy environment, but the measurement is made on the entangled partner which is in a low noise environment. This allows the measuring precision way beyond the noise floor of the measured environment.

I'm not sure what breakout tech will come along; I know that beyond about 300ly, even powerful signals like the original Olympics broadcast in 1936 would be below the noise floor of the CMB. But that is not really a hard limit to detection, the GPS signals are below the noise floor yet they are used everyday by billions of people.