"We only go ‘public’ when we want the ‘target’ to ‘know’."
The target's room, not Q's. Q certainly did not want o leave a clue as to who he was. And what better way to let the target know, than to enter the target's room clandestinely while the target is away and taking a picture while there? Send shivers down the target's spine.
It's reminiscent of when the Old Man of the Mountain (the founder of the Assassins sect) left a dagger embedded at the guarded bedside of a hostile leader to intimidate him with the knowledge the knife could just as easily been left in his heart.