Perhaps a universe at maximum entropy where even atoms and photons no longer exist - a more or less perfect vacuum. Would that be nothing? Though I suppose even a region of basically nothing would still have quantum fluctuations and wouldn't technically be nothing.
No one knows what existed before the singularity. And we don't really know that a condition of nothing ever existed. It's just an intuitive assumption of, "Well, it just had to be," from our collective imagination. In Christianity, there has to be a nothing, because the doctrine needs nothing so that something could spring forth from it. If something always existed, a god would have nothing to create. The stuff would just already be there, but possibly in a different form.
Like many others, I only know enough quantum mechanics to be totally baffled by it. But like you, I wonder if what we are talking about may be one of quantum's secrets, yet to be understood. It seems to be. At least, on a small scale we can observe a "something from nothing and back again" phenomenon happening. But we can't truly say that the particle that shows up came from nothing. It may be coming from something that we just can't see or understand, something that looks to our limited minds like nothing, something like a quantum fluctuation.
But I have a suspicion that an absolute and ultimate nothing, does not exist, may never have existed, and might be impossible. In a sense, a discussion of nothing might be much ado about nothing." But to claim that nothing is an original default state existing prior to something, just seems like reaching for something we want to be real only because we can't imagine otherwise. Mankind has traveled that road many times before, and I suspect we will make that same mistake many times in the future.