PARIS – The Hôtel Ritz Paris, famous for its bar, its swimming pool and its assignations, had a treasure hiding in plain sight, an exceptional painting that had been hanging on a wall for decades without anyone paying it the least attention.

With the hotel shut for renovation, the auction house Christie’s announced this week that art experts had decided that the long-ignored canvas was by Charles Le Brun, one of the masters of 17th-century French painting, and that it would be put it up for auction....