The drawing appears to be the rear view of a soldier sitting on a hill, holding a rifle. The Rousseau-esqueness made me hope it was drawn while Marie Antoinette was mistress of the Trianon but, as relaxed as she and her friends were there, they probably weren't relaxed enough to draw on the Royal walls. Even hidden Royal walls. Obviously, they weren't likely to draw a man with a gun, even one in a pastoral Trianon-inspired setting. This seems like a Citoyen-soldier, alone and unregimented, a concept whose time hadn't come while Marie Antoinette and her couterie played billiards and cards within these pastel walls. Maybe the Citoyen-soldier is guarding the Trianon.
Maybe this was drawn during the period that the Revolutionary authorities and even the populace had access to the Trianon during the early stages of the conflict. Or maybe the tour guides sketched it out, quickly, a month before our tour, because their tour had gotten stale and they wanted to spice it up.