属于省略。

张道真《英语语法大全》下册，第 388-389 页：

4．there 或 there be 有时可以省略。
（There）Must be something wrong．
（There）May be someone making trouble．
（There）Won't be anybody getting interested．
（There）Should be no time left for arguing．
I will pay you in three days，and（there is）no mistake．
（There is）No fool like an old fool．
（There is）No offence taken when none is meant．
（There is）An eye for an eye，and a tooth for a tooth．
（There is）Notelling what he is doing．
（Is there）Any news？（There is）Nothing particular．

—在简短的通告、标题、电报等中（省略 be）

Shoes（are）on sale．
Every sort of metal construction（is）undertaken．
English（is）spoken here．
No dogs（are）allowed．
Admission（is）free．
Rosy prospects（are）now yours．
Result（is）encouraging．
Paris Talks（Are）Hopeful But Delicate．
Father（is）ill．
Johnson family（is）coming．
Help（is）wanted，skill（is）not necessary，female（is）more desirable，salary（is）high．

所以，网友的句子可以还原为：

someone (was) shot in an alley, there (being) no witnesses. 独立主格结构作状语。

或者

someone (was) shot in an alley, (and there was) no witnesses.

或者

someone (was) shot in an alley, (with) no witnesses.