Art Thefts in Paris - Mathematics Professor Alfred Dunningham, PhD, #3

A seemingly well-organized series of break-ins occurred on the same night there was a city-wide museum workers' strike in Paris, France; a coincidence or an opportunity? The famous Musée d'Orsay was hit the hardest with most of their van Gogh paintings and Degas bronzes taken from their "theft-proof" frames and cases. Then the artwork was being sent back, and there were Greek letters painted on them.
Why would they be returned, and what did the Greek letters mean?Mathematics Professor Alfred Dunningham, PhD, noted for solving mysteries with mathematics, is brought to Paris. He determines that the Greek letters also mean numbers, leading him to a special 11-digit sequence, and the French National Police rush to the Pere Lachaise Cemetery where more bronzes were found. The Police know whom to nab, but their suspect is nowhere to be found; was some advance notice given?