Surely you can't hide something like this and we would see it in the news very quickly after a launch?
If ballistic missile launches can be hidden we might as well go home. First strike for everyone.
Well not really. Usually the host nation might report on it (India with Agni Prime recently) or western media would pick up on it from host nation. Foreign intelligence almost rarely publicly leak this to media and unless foreign states comment on it officially, something like that would be missed. I mean we don't exactly have our own AEGIS ships, early warning radars near China or our own launch detecting satellites. So how would we know. The foreign state/s that usually disclose this type of stuff in annual papers may not choose to or may not know enough to report on it or may not have yet had the time to print it. Doesn't mean it didn't happen.
JL-3 tests were leaked by Chinese sources and then sort of verified by pseudo accurate and pseudo reputable/official sources. If it weren't for leaks, we wouldn't have known about it.
They can detect launches and observe them if they're stationed near enough to watch but they won't necessarily report on a new Chinese ICBM. It's need to know until it isn't.