Visible crew/equipment: While Moretti is in the barber shop on the phone with Sonny, the camera tracks closer to Moretti, Sheldon, and Carmine, and as Moretti says, "Come on, let's be reasonable...just give me your name," we can see their three T-marks (dark chalk) on the floor at their feet. (00:28:40)
Visible crew/equipment: When Sonny wants to use a heavy desk to block the bank's back door, Mulvaney removes his suit jacket, and as they both push the desk toward the door, the outline of the actor's Lavalier mic transmitter is visible at the lower right side of Mulvaney's back. (00:24:15)
Visible crew/equipment: When Leon is brought to the scene from the hospital and escorted toward the barber shop across the street from the bank, Sonny shouts, "Hey, Leon! Over here!" In the next shot facing the crowd around Leon, the production crew's camera crane becomes visible in the window reflection at the right side of the screen, as it films this shot. (01:05:00)
Visible crew/equipment: When Stevie gives Sonny the handgun, as Sonny tucks the handgun into his waistband, the actor's mic wire becomes visible. After Moretti's first phone call, Sonny is angry that "Jack downtown" gave him wrong information, and when he stands up there's an odd bulge at the back of his pants, presumably his mic transmitter. Then Sonny asks Mulvaney about the back door's location, and when Sonny leans down to tell Sal, "Cover them, cover them," we can see the mic and its wire between Al Pacino's tie and shirt. In exterior shots, we can see the outline of the white tape under Pacino's shirt (holds his lav mic in place due to the actor's frenetic movements). (00:07:55 - 00:20:40)
Answer: Because Sal was in the bank with his gun.