Piece of the Puzzle

Now see, I'm no idiot, and I realized that these cases matched up in a weird way. For instance, why was the gun in Angels and Ricks car? That was something that really bugged me. This murder was solvable. Everyone else was just saying that it wasn't.
I returned to the scene and scoured entire place looking for a clue as to where the gun went. I used the UV light and black lighted the area, looking for a hit as to how the gun managed to leave the room. After about fifteen minutes I found a trail of flecks of dried blood, leading from the point of origin (or where the gentleman was shot) to the third story window of his apartment.  Why the hell would it lead out the window when the window was locked... upon closer inspection I found that the window was not locked. Just shut.
I climbed out of the window on to the fire escape and looked around. someone could have easily hoisted themselves up to the first story ladder, then climbed up to his window, walked in and taken the gun, walked back out, shut the window and climbed back down. "To what end?" I said out loud.
I climbed back into the room and checked the trail again. Sure enough, it went from the chair, to the floor, to the window sill and .... gone. Somehow managed to end up at a scene six miles away. Guns can't walk so how did it do that?
I excited the apartment through the escape hatch, I'm the police so I can get away with things like that, and on on my way down I found a heap of string. Bloody hell, it had blood on it. That would explain the missing bootprints in the apartment. The murderer had the gun attached to a string, and after the blood had dried somewhat, and before I arrived to do my analysis, had nabbed the gun and disappeared. Well, no, not disappear entirely. The gun ended up at that scene six miles away, a few days ago. Nothing seemed to add up here.
A few days later, as we were cleaning up the crime scene, a call came in and I picked up because I happened to be near the car. The dispatcher sent me through, it was a personal call to my personal line at the station. The citizen was wondering if anyone had turned in his gun, someone had broken into his home about a week ago and stole a number of guns. He read me the serial numbers, and with one of them, my heart jumped a beat. The serial number that matched the small handgun that his father had given him as a birthday present when he turned 21, that was light and easily hid in a pocket book, that a woman could get away with having in her bag, was the same one that we had in police custody downtown.
I stopped the kid right there, "Listen Dave, we need you down to the station right about now, do you think you could make it? We have one of your guns."
"Really? That's great! When do you want me to come down?"
"Right now kid, right now."