Gus had Tyrus sweeping for bugs because he suspected (as Walt's plan anticipated) that Hector had become a rat and allowed the DEA to bug his hospital room in order to entrap Gus. Sweeping for bugs would only detect a bomb with a radio transmitter (like last episode's remote-triggered car-bomb); it wouldn't detect the bomb itself (or other simple, non-transmitting electrical devices). The bomb Walt planted was detonated with a simple electrical circuit, which was closed/activated by Hector dinging his bell. There was no radio signal for Tyrus to detect.