Encryption can't hide the fact that there is a signal. Sufficiently secure encryption makes it so that any ciphertext you receive is equally likely to have been

Encryption can't hide the fact that there is a signal. Sufficiently secure encryption makes it so that any ciphertext you receive is equally likely to have been produced by any plaintext. But if you're able to observe that some ground station on Earth transmits radio to the same point in orbit, at the same time, every day, and receives a response, the fact that that is happening can defininitely be distinguished from any natural process. By the same token, if humanity disappeared but left a bunch of encrypted hard drives around and aliens came and found them, they may have no hope of distinguishing the bits from random bits, but they'd be able to tell that no natural process encodes any kind of bits at all onto magnetic discs or flash drives or whatever else we manage to come up with. The actual issue is what the other commenter said about the impossibility of detecting such low power radio at all.