Like, say, you are a revolutionary who’s imprisoned, but you are allowed to write letters to your family. How would you covertly send a message to your fellow revolutionaries that only they can decode? (Your family are on your side will pass the letter along)
Obviously, it cannot appear to contain any obvious ciphertext, since the prison guards would just rip up your letter and not send it.
As another user mentioned, I think the safest way to do this in the scenario you described would be to establish a code where innocous sentences have a certain meaning attached to them in advance, but that of course would require the hypothetical person to be prepared/expecting imprisonment.
Another thing I can think of would be smuggling a small phone in the ass or another orifice, but that would be a lot more risky, as it’d be likely that they’d check every square millimeter of the person’s body and being caught probably wouldn’t end well.