Visible crew/equipment: When Sweet begins talking to the pimp after pulling him over, a boom mic is reflected in the front windscreen. (00:43:50)
Visible crew/equipment: In the scene just after the pimp is shot by the cop in his Cadillac, the right side of the car is shown as the cop walks back to his motorcycle. The filming crew is visible moving around in the reflection of the car paint of the right side quarter panel, in spite of the paint being dulled to reduce the reflections. (00:46:25)
Visible crew/equipment: Near the end when Harry is running from the motorcyle cops, he runs up a ramp and through a door. Below the ramp and just above the wooden wall, a camera and crewman wearing a white hat are visible. (01:46:25)
Visible crew/equipment: When the car crashes into the office building after Harry fires a bullet through the car windscreen, you can see the film crew sitting in the background.
Visible crew/equipment: After Callahan removes the bomb from his mailbox, he looks up at a man on the nearby stairs. As a car drives by on the street, the lights from it cast the shadow of the boom operator onto the wall.
Visible crew/equipment: After the pimp is stopped by the policeman on the motorcycle and shot, the camera pulls back showing the pimp's arm lolling through the car window and the reflection of the camera crew moving about on the back, driver's side wing.
Visible crew/equipment: When the corrupt copper pulls over the pimp, there is one shot where the camera is zoomed into his glasses. You can see the camera and 2 crewmembers reflected in them.
Visible crew/equipment: The camera and cameraman are reflected in the glass in the shooting practice room.
Visible crew/equipment: When Harry returns to the trashed car after the last motorcycle cop was killed, he leans into the back seat to look for the bomb and Briggs pulls a gun on him. At this point, a small boom microphone is reflected in the window behind Harry.
Visible crew/equipment: When Harry is running up the steps to enter the aircraft carrier, you can see a camera and crewman in a white hat below, just above the wood wall.
Answer: Because Davis and Sweet demonstrated keen interest, skill and knowledge of firearms (particularly of Harry's .44 Magnum) when they first met. They were flattering Harry, probably in anticipation of inviting him to join their death squad. Harry was impressed and asked for them specifically, even though they were rookies, but he didn't suspect them of being the vigilante killers at that point.
Charles Austin Miller
Yes, he did suspect them. In fact, he suspected Davis from the moment he found out that Davis was the one who reported Charlie McCoy's death. He confirmed his suspicion about Davis after he compared similarities between bullet that killed one of the victims and the one he purposely misfired from Davis' revolver during shooting competition and retrieved it afterwards. He explained it all to Early Smith right after raid on Palancio (and thus unwittingly sealed his partner's fate).