In Spenser Confidential (2020), Globe journalist Wayne, shows Spenser CCTV footage from a convenience store that details Gloria Weisnewski’s murder. Wayne admitted that Globe put him on another story and is avoiding Gloria's murder.
Spenser: So what're they hiding?
Wayne: I was hoping you could tell me that. This is fucked up, man. Tracksuit must have put them on it.
Spenser: Fuckin' Boylan.
Spenser recalls the incident where he and Driscoll were sitting in the parked police car near Captain Boylan's house with no police cars around.
Driscoll: Hey, one flash behind. One flash behind you, baby.
Spenser gets down from the car, walks around Captain Boylan's house, and knocks on Captain Boylan's house door. Boylan answers it. I know flash refers to "cops flash their side lights".
Why did Driscoll say, "One flash behind you"?