Wikipedia references a book named The Game of the Foxes referencing Polish control of an Enigma machine and the plot involves a character cracking it.
4 years before Władysław Kozaczuk wrote in Bitwa o tajemnice that the Poles had successfully decoded Enigma messages
In the same year (again according to Wikipedia) "David Kahn in The Codebreakers described the 1944 capture of a Naval Enigma machine from U-505 and noted, somewhat in passing, naval Enigma messages were already being read."
I infer that the idea of cracked communications had entered the public consciousness enough after 1967 that it inspired novels to include it. Did anyone surmise the communication breach previous to that year? Were there any "Carrots improve your vision and our British pilots eat lots and lots of carrots" hand waves that explained how clever the Allies were without disclosing the existence of Ultra?
So a couple of things on this. First, the existence of Ultra was officially revealed in 1974 but the concept of codebreaking and codewriting was very much a part of popular knowledge prior to that. So it's well within reason that people who thought about these kind of things would've known or guessed that the Allies had at least partially broken the Germans' codes during the war, but perhaps not the how. Honestly, it was such an under-discussed aspect of the conflict that public consciousness probably never really dwelled on it very much. When your average citizen thinks of a war, in my opinion, they don't think about codebreaking. As far as the need for handwaving techniques, it was largely unnecessary because of how enigma information was used and the way in which the Germans acted.
The Allies were incredibly cagey about the use of Enigma code intelligence and how it was distributed. In short, if someone was not in the "need to know" category regarding Enigma-related intelligence, they weren't going to hear about it or make use of it. The Allies very intentionally managed what intelligence they used to ensure that their hand was not tipped.
Furthermore, the Germans were seemingly oblivious to the fact that their code was broken and the individuals responsible for code development with Germany were few in number and relatively silent throughout the war. This made it less than necessary for the Allies to start creating catchy little myths about how they were responding to German movements. If anything, spreading those sort of rumors may have drawn unnecessary attention to the fact that the Allies were doing such a good job of second-guessing the plans of the Germans.
This is a very good answer from tayaravaknin about how the Germans didn't acknowledge that their code was broken: https://www.reddit.com/r/AskHistorians/comments/1yn9na/did_the_germans_really_not_know_enigma_had_been/ which really highlights why such deception was just unnecessary in context. With the Germans seemingly oblivious to the Allies' intelligence coup and the information so tightly controlled, it just wasn't important.