Not really, actually. I am kinda astonished how large you think that timespan is in which she went out while Battler was still searching the room. To take matters more clear, because apparently the chapter break blurred things, I will recount (once again) what had happened after Mashu’s run-in with Beatrice:
After Battler found Mashu in the attic, they went downstairs and met up with Ronove and Nitocris, together they went to check the bedroom of Ritsuka and Robin. The door was locked and had to be unlocked by Ronove. Inside the room, Ritsuka’s corpse didn’t exist in any place whatsoever. And they all saw that the window bars were still intact.
Wait… are you building arguments from the basis that Servants have super-speed? I just realized that I actually haven’t thought about that at all. Just to prevent you from going that way: If Nitocris or any other Servant would have used their magic to destroy the corpse, it would have ruined the carpet. The carpet everywhere in the mansion is fine!
Now to your second blue truth… damn, I really should phrase my reds in a way to deny you more broadly so that I won’t have to do this.^^
After they found the corpse of Ritsuka, Ronove locked the door. It should be obvious that even if he had been alive, he wouldn’t have been able to just sneak out. I should also note that Ritsuka is dead.