From Futurism, we have an article on the latest theory about the EM drive.
In short: the drive is not reaction-less, but emits photons. They’re not being detected because they’re coming out of the drive in pairs with opposing phases. Those photons escape the EM drive’s cavity because the phase conflict eliminates the photons’ magnetic field — which also means that the photons can’t be detected by normal means.
Obviously, add “theoretically” to most every sentence above. The cool thing about this theory is that it fits the observations and doesn’t seriously challenge what we already think we know about physics.
The monkey in the wrench? It’s finding those emitted photons to prove the theory. In the Daily Mail’s version of the article, the author suggests that they’ll need an interferometer to do it. (That’s the same device that was just used to detect gravity waves.)