Round here it means somebody's missing a pair of shoes.
I suppose culture varies from Essex to Los Angeles.
I dunno...that's what it means here, too, and I'm only an hour from LA.
Okay let me rephrase that:
"The prevailing notion that tennis shoes dangling from power lines mark the location of houses where drugs can be purchased. Or that they mark gang territories. Or that they mark the spot where gang members have been killed."
That sounds like a reference to street gangs.
Dudes do that to show that "this is their trap!"...or they're just being dicks with people