Unknown event id

When I was testing my RC range my Quadcopter went into RTL mode with the message [Unknown event with id 21936010]. I have found how the event IDs are constructed, but not how I can find out what the event ID means. Is there any way to find what event would trigger this message?

hi. i am also getting issue with the (unknown event ID with 3307008) does anybody knows how and why these event ID created and what does that means?