Treasure Keys are a type of loot found in Sly Cooper and the Thievius Raccoonus. Each member of the Fiendish Five, excluding Clockwerk, has hidden them in seven trophy cases in the far corners of their environment.
You will start in a mission that ends near a way to the hub, at which you will do a little moving to get there. The first key is near the end of the zone's start. You will get keys for reaching the end of a mission or you'll find it as a prize.
Three to four keys will give you access the other half of the hub, while seven keys will grant you access to the boss of the hub.