Just like the Vela Incident salilites would pick up the explosion,seems unlikely another country wouldn't confirm the explosion.

What this source told me was that , and again I’ve never found this source to be wrong, was that there was actually a missile fired. The missile was brought down by the United States … antimissile program…. It was a submarine based missile and … it was from a Chinese sub…. This was a renegade faction within the Chinese military that was in charge of this sub that fired this missile, and in fact the Chinese Navy, supportive Xi Jinping [Current President of China] was the ship that then took out that submarine.