The item was dark, and right up against the wall of the safe which is black. We didn't notice it, and apparently neither did the cabin attendant who previously serviced the room. Probably also why the owners missed it when they were cleaning out the safe for their departure. I suppose it's possible that someone snuck into our cabin after we checked in, broke into the safe, and stashed something there, but I'm thinking it's probably not the most likely scenario.
As we were leaving our cabin, 10309 on Symphony of the Seas this morning (11/17/2018), we noticed some items left in the safe by a previous occupant of the room. If you stayed in the room and are missing items, please contact Royal Caribbean. We notified the attendant of the items and she said she would take them to her supervisor.