Let's say we do detect encrypted signals.

We can't.

The Holy Grail of encryption is to generate a signal indistinguishable from random noise, and present-day ciphers with large keys sizes (>512 bits) already reduce nonrandomness to parts-per-billions. As Greg points out, any patterns that remain in the cyphertext provide leverage for codebreakers. A strongly-encrypted carrier would be a hash resembling the constant radio noise from Jupiter, and we'd be unable to determine whether its origins are natural or artificial, no matter how strong the carrier.

(Hmmmm....I wonder if the NSA has ever taken a look at the signal from Jupiter?)

Conversely, natural objects like pulsars can generate non-random signals. Is that metronomic tick a spinning neutron star, or a node in the Galactic Positioning System?...hard to say.

--Robert