I think that actual tight security is in place for the facility, not the planes themselves. They placed the CAC smack dab in the middle of one of the most populated cities in China with an estimated population of at least 10-12 million. We also know Chengdu is a major international metropolitan where many many foreign companies and personnel live and work. And they do flight tests and fly their J-20's all over the city. With all the noise and things, it would be almost impossible to ignore it even if you want to. I can almost guarantee that people working and living nearby the CAC must all be quite annoyed by the J-20 by now. So what is the purpose of tight security if their goal is keep the J-20 a secret? IMHO, the actual goal of the tight security is the facility itself, not the plane.
they have plenty facilities in remote places. If they want to hide the J-20, they have plenty choices.
I think that they never intended to hide their new planes when they designed and built the CAC at the very beginning. Chengdu has been a major city in China for at least 2000 years since the Han dynasty. So it's not like they built this facility in the middle of nowhere back in the day, but Chengdu eventually exploded into a major city as it is now. So they knew fully well that they were building the CAC in a "high-risk" area, where it would be impossible to hide anything, especially planes flying around in the sky.
Nope.
Have a look at a map of Chengdu.
CAC is nowhere near the middle of Chengdu, it's far to the North West of downtown Chengdu, about 7.5 miles away as the crow flies.
Just to put that into perspective, I live about half that distance to a major international airport, and I can't hear a thing unless a plane is flying towards my house.
CAC would categorically NOT be flying experimental jets over a major metropolitan area, any short test flying would be done over the western areas of CAC, where there are far fewer residential areas.
For anything fancy or long range, they will head over to a dedicated test site.
I would bet good money that the overwhelming majority the 10-12m residents of Chengdu has never actually seen a J20 with their own eyes. It will only be those who live a mile or two along the J20's preferred egress routes that will be familiar with it, and I think they would have planned it so that as much of that route is over sparsely populated areas as possible.
As much as Chengdu has developed, it is only now just encroaching on the Northern and Eastern boundaries of CAC (I believe the blue roofed city yo the west of CAC is part of its Campus, and would be considered a secured area as well for support institutes and family housing for CAC employees).
That's how they were able to keep the J10 under wraps for all those years. Back when it was being developed, civilian population centres were far enough that they can easily avoid being detected from the ground with a little careful planning.
When CAC was first set up, it would have been plenty remote to offer good security, yet still be close enough to civilisation as to not make it too much of a burden for the workers.
Boeing, LockMart, BAE, Dassault, pretty much ever major aerospace powerhouse is built not far from major population centres.
There are remote top secret research and testing facilities, usually deep in the deserts somewhere, but it's both unnecessary and unfeasible, not to mention undesirable to put all your weapons related research and manufacturing centres in such places.
By its very nature, such remote places are few and far between. Requiring all your weapons research and manufacturing facilities be placed in such remote locations would in all likelihood create an unhealthy concentration of all your most important weapons facilities in a few very remote areas. Making them extremely tempting and vulnerable targets for nuclear first strikes.
You also have to consider that for most countries, their primary security concern isn't with nosy locals, but rather foreign nation state intelligence gathering.
In that respects, having major weapons facilities near major population centres can be a massive benefit, since the vast amount of regular civilian road, rail and air travel around such busy cities can go a very long way to helping camouflage and hide military traffic.
Whereas in a remote region where hardly any locals live, and almost none of them have cars, even a small convoy of military vehicles would stand out like a a sore thumb.