Sentry-bots were the robots created in the same factory that Clank came from. Clank was actually a defective Sentry-bot. Ratchet could use the Hologuise to get pass these robot guards on planet Quartu and Drek's Fleet. If Ratchet was not wearing his Hologuise, they would either shoot fire balls at him or press an alarm button to activate fort defenses for almost immediate termination. If Ratchet waved at them while using the Hologuise, they would wave back and deactivate forcefields so that he could pass.
In Going Commando, Clank mentions them to the Mutant Crab when stating that like him, he is also a defect from a series designed to destroy. In Ratchet & Clank Future: A Crack in Time, the Sentry-bots from appeared on some of the memory screens in Clank's subconscious.