Also any zero propellent drive is also an over unity device. Easy to prove.
If such a thing has actually been discovered, the very likely result is that it's neither "zero propellent" nor over unity, but instead has something being emitted that we don't understand.
For example, one could imagine an engine that seemed to have no propellant, but was in fact creating and emitting dark matter. On the lab bench it would be consuming energy that would be going somewhere mysterious (e.g., not heating up enough for the energy inputs), generating measurable thrust, and having no measurable propellent. Obviously that's not what's going on here, but something like that (emitting a propellent we don't know how to measure) would be the only rational explanation for any such device.
It's also theoretically possible to have a "warp" drive that produced thrust without propellent by altering the local spacetime metric. But this would not be "over unity", would be quite obvious as it would be turning local space into a lens, and likely isn't actually possible, for all that the math allows it, as you'd think we'd have seen evidence of it by now.
I guess "negative mass" drives also aren't ruled out yet, which also would have no propellent and while they are perpetual motion machines, they aren't "over unity" due to a technicality. Negative mass seems even less likely to be actually possible, despite the math allowing it, given the lack of evidence of its existence.