While radar warning systems first appeared back in World War II, these devices have become much more capable over the last 70 years. No longer do they just warn the user that radar signals are being beamed at them. Now early warning radars can tell you the exact direction, distance, and type of radar involved. Some warning devices can jam or deceive the radar. These additional features require, or work better with, detailed information about enemy radars. Because of this, over the last few decades ELINT (Electronic Intelligence) has become more important. This means trying to be around to record the transmissions of radars used by potential enemies. You don’t want potential foes to know what you have collected or what you can do with it.
Japanese radar warning gear has apparently detected other instances of Chinese warships switching on their weapons targeting radars against Japanese ships or aircraft and only mentioned that in passing during the current situation.