1) That some form of propellantless propulsion may in fact be 'a thing'.
2) That exotic propulsion technologies may be 'born secret' by default — even if you or I invented them — because of their national security implications (can be formed into an ICBM platforn). There would be no publicly-referenceable classification list informing you of this, because that is not how it works and would defeat the purpose. Our best precedent on this is in fact the Manhattan Project. Living then we would not know that isotope fission methods were restricted data, even while speculation could be bandied about in amateurish publications (cit. Alex Wellerstein's excellent blog https://blog.nuclearsecrecy.com/2014/03/07/death-dust-1941/ ) Moreover, we would see those few scientists who were actually pursuing relevant study under government auspices actively misrepresenting (i.e. lying) to the press. cit. Urey on heavy water:
"...it got Harold Urey, the guy who discovered heavy water, to write a letter to the editor saying I don’t have any clue what this is about. There is certainly no way to use this in war, which is a total lie."
https://ahf.nuclearmuseum.org/wp-content/uploads/2014/06/Ful...
And then it is a very long and hard slog to get from basic research to something practically usable. Even if at some point you start classifying it, other teams can follow the same path openly in parallel.
Something is unlikely to be "born secret" unless it is developed in a government lab like LANL or LLNL. Are those kinds of labs even working on spacecraft propulsion? And even if they are, are they likely to be getting ahead of open research (academic, private sector, non-classified government labs)? Being classified can actually slow research down, by adding bureaucratic red tape and removing opportunities for collaboration and getting more eyeballs on a problem.
This sounds more like something out of fiction than reality.
The Manhattan Project was different because it was during a World War.
