Whispers From Outer Space
If there are other alien spies in the galaxy then where are they? Maybe they are there and we're just not detecting them for some reason.
If interstellar travel turns out to be extremely difficult than sending out messages is the only way to establish first contact. If the aliens aren't purposely sending out signals to be detected than any signals that leak out are likely low power and would be highly attenuated over more than a few hundred light years. Dust and signal noise from naturally occuring RF would further dillute the signal.
Technological aliens probably wouldn't broadcast using frequencies close to those that naturally occur as random noise. They would also probably encrypt and compress their transmissions.