Visible crew/equipment: When we see Nancy, Robert, Dale and Brennan on the boat, as the camera pans past Robert and Nancy, you can see the camera reflected in the shiny post of the boats wheel. (00:14:55)
Visible crew/equipment: At the end of the singing car scene, when the range rover swerves around oncoming traffic, you can see a police officer stopping civilian traffic in the background of the shot. (00:24:25)
Visible crew/equipment: After their argument in the car (after Brennan and Dale have shown their presentation), Robert rushes into the house. As the camera moves backwards, you can see a crew member reflected in the mirror on the wall. (00:57:10)
Visible crew/equipment: When Brennan and Dale are talking about the drum set in the living room, you can see a crew member on the glass window.
Visible crew/equipment: When Brennan and Dale are on the stage at the wine mixer, Brennan sings opera and when Dale plays the drums you can see a crewmember reflected underneath the cymbal.
Answer: I doubt it's a person. Jack likely refers to Jack Daniel's, and a handle is usually referred to as a half gallon. Hence the name Jack Handle.