And do not think it could happen without that. Such a test would have to be announced in a maritime environment so that other vessels would steer clear. The US announces these types of tests regularly...and the live fire tests China has done with its other weapons have also been.
And when they do, the monitoring systems various nations have in place will observe it...just as China observes what the US does.
Just playing devil's advocate here, but a couple of scenarios for you to consider.
1) Why does China have to conduct an AShBM test by itself? If China wanted to disguise such a test, as it would be in China's interests to do so, it could conduct AShBM tests under the cover of conventional missile tests.
All the warning and cordoning procedures would be similar and met. You'd just be shooting off some AShBMs in addition to the conventional missiles advertised.
China could further confuse observers by conducting an AShBM tests along with amphibious assault exercises by parking a few target ships close to an island its forces are staging a simulated storming against (which in itself would not be an unrealistic scenario to practice, and has a lot of merit for doing, for example testing missile seekers against background clutter interference).
It could fire off conventional IRBMs at land based targets on the island, while at the same time shooting standard AShMs at the seaborne targets.
Choose an island with large cliffs, park a target ship in from of the cliff facing the mainland, and that should effectively shield the ship from any standard observation assets the likes of the US and allies would deploy so they would not be able to tell what hit the ship.
Even if they observed terminal manoeuvring on the incoming BMs, that would likely be chalked down to anti missile-defence penetration manoeuvring.
Conduct this exercise in the Bohai region, which is enclosed by the Liaoning and Shandong peninsulas, effectively making it a contained Chinese sea, where outside observation assets cannot get within dozens or even hundreds of miles of the test zone, and they will have a fair chance to keeping any such test under wraps.
2) Why does such test necessarily have to be conducted out at sea? China could conceivably place a target ship in one of its major inland lakes to target.
Given the short flight time of the missiles, and the fact that China can monitor foreign observation assets, they could conduct the test at a time when no one else has anything overhead or pending, and have everything cleared up or relocated before anyone can re-task space based assets to take a look.
Place the launcher between the lake and know standard ballistic missile test ranges, and it would be very easy to dismiss such an incident as a failed conventional missile test.
That's just two scenarios off the top of my head.
The point being that if China really wanted to, there are plenty of ways it can hide or disguises such a missile test such that it would be hard, if not impossible, for the most independent and diligent intelligence analysts to be able to conclusively say what happened.
There is value in staging unambiguous, highly public demonstration tests, and I think the PLA will do exactly that if tensions risk so high that direct conflict looks like a real possibility.
But there is also value, and methods, in which even in this day and age, the PLA could still hide the true nature of top importance tests from foreign observer with some effort and smart.