If you've ever been to the Louvre, the size of it is mind-boggling. The main palace (not counting the modern addition) is 2.6 million square feet. Bigger than Versailles, bigger than Esterhazy Palace. The only museum in the world that's bigger is the Hermitage in St. Petersburg. The Louvre is like 10 Wal-Marts broken up into long corridors.
So it's not surprising that there aren't enough cameras or sensors. Security for a building like that would be crazy expensive. On the other hand, the Louvre costs 22 Euro and takes in 9 million visitors, so that's roughly $200m just in admission revenue. I'm sure the French will secure it now that the vulnerability has been exposed.
Perhaps they should hire Walmart as security consultants.

