I suspect, if they exist, they either pair bond or live solitary lives similiar to tigers or bears. Reptiles are different from mammals in breeding. I believe there are even female reptiles who do not need males to procreate.
If they exist, it's no a given they are threatened. A pop. of 1000 could conceviably have only 250 breeding females. Still enough to breed without inbreeding.
I'm not sure about that. Given that they decide who to breed with on an individual basis, inbreeding would be very very likely. In such a small population, I would expect there to be either a very large litter or a very prolific breeding cycle. The other aspects such as loss of habitat, disease, toxins in the food chain, would I suspect result in an extinction.
Also based upon the taken for granted aspects, bipedal humanoid, their biology would be much more like a primate than a bear. And so, naturally their behavior and pathology would as well.
And so that would be like saying a thousand Neanderthals are living undetected in North America. With the capabilities of hunters now a days, easily track, and kill anything to extinction if allowed to, there is such a low possibility for something to go so undetected as to not even leave a single bone, or a single carcass open to discovery, it's far more logical to say there is no such thing rather than to insist on the possibility that there is.