I think the leak is Gabriel. Brenda Lee, Gabriel, and one other detective, (Tao, I think) were the only ones in the car with the guy they dropped off in the neighborhood to be killed. Gabriel has a prior relationship with Taylor from before Brenda Lee came to LA. Taylor gossips.
Also, the actor that plays GabrielClick to see Spoiler:is the only one not returning in the series with Rador.