Q. What is it supposed to be on the show? A. An alley.  Q. Where is it supposed to be on the show? A.  In or near Miami, Florida, right behind the pool room, Ernie's Taproom.  Q. When did we see it on the show? A. We see the alley  in Episode 3, of Season 1.  After loan shark “Handsome Tony” Ferrar comes to collect his weekly payment, and finds that Dexter has gambled it away playing pool, he flashes a gun and forces Dexter to go outside with him to the alley behind Ernie’s Taproom. Once outside, he pulls the gun and points it Dexter. Dexter behaves as if he is terrified, begging Tony not to shoot him, but Tony fires the gun, deliberately missing Dexter’s head by only inches. (That’s the same thing Tony did with Rene, the young busboy from La Palmita restaurant, killing the boy’s mother who was in the next room.) As Dexter cringes on the ground, Tony points a gun at his eye and tells him he will kill him if he doesn’t have his money by next week. Tony leaves, not realizing he has been played. The bullet he fired embedded itself in a tree trunk behind Dexter, and as soon as Tony is gone, Dexter calmly digs that bullet out of the tree.

Later, Dexter uses a ballistics microscope at Miami Metro to compare that bullet with the one taken from Rene’s mother, proving that Tony fired the shot that killed the woman.

Q. Where can I find it in real life?

A. Not only is it not in Florida, it's miles away from Ernie's Taproom. 
 
This scene was filmed in an alley behind Vitello's Italian Restaurant, which is located at 4349 Tujunga Ave, in Studio City, CA.

This is in the San Fernando Valley, about 40 miles north of the bar that served as Ernie's Taproom.

So why would they travel 40 miles rather than simply film it behind the original bar?

I can think of two reasons:
 
First, the original bar doesn’t have an ally behind it (only a small side parking lot), or a rear door, or a big tree for the bullet to hit.
The alley behind Vitello's Restaurant, on the other hand, has all three of those elements.

Second, they didn’t have to travel 40 miles. TheIr new studio is also in Studio City (in fact, it is the studio that gave that city its name), and that's where they routinely work. 
 
And the restaurant is just 4 miles northeast of that studio.

Q. How the heck did you figure out where it was?

A.  I didn’t have to figure this one out, I found out via a helpful tip from someone in the know. 
At first, I thought they were filming inside Vitello's Restaurant, but when no such scene appeared on the show, I took a closer look and recognized the alleyway behind the restaurant.

Checking out the StreetView of the alley, there was no question that it is the same place. Everything matches: the (left) door, the dumpster, the white utility box on the wall, the weird wiring above, and the big tree.

About the only changes they made were to temporarily move that small white shed out of the way, place a small sign on the door, put two fluorescent light fixtures above the doors, and add a few trash cans.

Here is a matching StreetView of the alley from Google:

 
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