D'Angelo tap, tap, tap murder reference

In The Wire, Season 1, Episode 13 (The Sentencing), there's a scene where D'Angelo (Dee) is in the interview room with his own lawyer (not Levy) about 19 mins into the episode:

PEARLMAN: Your client must realize that any agreement is dependent on his full cooperation.
DEE: Well, yo, there ain't nothin' else.
[McNulty pulls out a picture of Deirdre's corpse]
DEE: Deidre.
[Bunk knocks on the table three times]
DEE: Tap, tap, tap.
[Bunk knocks again]
DEE: She was one of my uncle's girls.
McNULTY: Yeah, but we got people who put you with her the night she's killed.
DEE: Yeah, I didn't know what he was gonna do her. I swear. They played me.

Something that confused me is how do Bunk and McNulty know about that tapping on the window which they gesture about while handing the photo to Dee (the tap tap tap reference)?

Dee talks about the tap tap tap when describing the murder to Poot, Bodie and Wallace in a previous episode (4 - Old Cases). The police aren't aware of this conversation. In the famous scene where Bunk and McNulty investigate the crime scene without any dialogue apart from "F*ck", they also tap on the window 3 times. How they would know that this happened I have no idea as no one witnessed these events and no evidence would suggest that the shooter tapped on the window 3 times (what could possibly suggest that took place?).

So during the interview, how did Bunk and McNulty know this tap, tap, tap event took place? Am I missing something from the plot or is this just some sort of story telling mechanic which doesn't fit in with the events?

plot-explanation the-wire

Best Answer

During the "Fuck" scene, Bunk and McNulty understood how the crime happened, especially that the woman was bending over while looking at the window (the bullet entered high in the chest and came out low from the back, at 3:18 in the video).

Something had to attract her attention, from the outside, for her being positioned that way. They deduced it was someone knocking at the window. I suppose the most common number of knocks is three, hence the "Tap, tap, tap".