I have a feeling that China is very prepared to invade with extreme haste and their intelligence agencies would not let something like this go unnoticed.
The sheer logistics of a large scale mobilization means that its obvious weeks and even months in advance. Moving troops, equipment and ammunition to staging areas etc. Even just preparing transports shows up.
Think back to the weeks before the Ukrainian invasion. It's kinda hard to hide it.
As far as I know, China has not been hiding this preparation. They are actively preparing to invade Taiwan, at least for the sake of appearances so that they can be seen doing so.
