Fortunately for you, I've launched these signal repeaters throughout Raleigh's fortress. Get close to one and I can checkpoint your progress.
― Bentley explaining the function of a signal repeater to Sly Cooper.
Signal repeaters were devices created by Bentley to assist Sly Cooper in his quest to reclaim the Thievius Raccoonus from the Fiendish Five. He set them up throughout the various locations that the Cooper Gang visited in Sly Cooper and the Thievius Raccoonus.
Signal repeaters served as checkpoints; should Sly suffer a fatal injury, he would re-spawn from the last signal repeater he activated, due to the device "repeating" the path he had already taken.
A signal repeater's design consisted of a device with a camera on it. Whenever Sly got near it, the device would activate and the camera would take a picture of him. At the same time, a holographic image of the Cooper insignia would appear above the device, which indicated that the signal repeater had already been activated.