I think you may be wrong - it DOES get activated on-screen when he is about to kill furniture on Star Destroyer. He holds it so we see the hilt on the right side of the screen, and the blade extends all the way across the screen.
Here's the WGA script for the scene:
The droid... stole a freighter?
Not exactly, sir. It had help.
Ren says nothing. Which says everything. Mitaka sweats.
LIEUTENANT MITAKA (CONT'D)
We have no confirmation, but we believe FN-2187 may have been helped
in the escape--
Ren IGNITES HIS LIGHTSABER, TURNS AND SLASHES AT THE CONSOLE BEHIND HIM!
HOLD ON Mitaka, who reacts, looks away -- winces.