Most likely the event you saw was at the Carousel du Louvre, which is the area with and near the shops, and includes a good size exhibition area. There are quite a few functions that return there each year-a major photography expo, an upscale wine event, etc.
Theess events are not in the actual museum that holds the world famous colection of art, and admission to one does not usually grant admission to the other.
One would assume that the Musee du Louvre (where the art is) is well protected. But this is France where theives walzed into the Paris Musee d'Art Moderne at 3 am a few years ago and walked off with a several hundred million dollars of art, so you never know.