A quick note on the tracing of black magic.
To quote from Love Hurts:
“I’m sorry, Harry,” Captain Luccio said. “We don’t exactly have orbital satellites for detecting black magic.”
... (cutting a paragraph because it doesn't have anything to do with point and I don't want to type it in)
“Too bad we don’t, eh?” I asked. “Unofficially - is there anyone who might know anything?”
“The Gatekeeper, perhaps. He has a gift for sensing problem areas. But no one has seen him for weeks, which is hardly unusual. And frankly, Warden Dresden, you’re supposed to be the one giving us this kind of information.” Her voice was half teasing, half deadly serious. “What do you think is happening?”
In that scene Harry knows that black magic is happening. He's seen the results. The only problem is that he doesn't know where the spells were being cast or by who. When he finally gets a general area he wanders around all day trying to sniff out things and nothing jumps out at him.
In short, minor acts of black magic are hard to trace. That's probably why most warlocks are crazy by the time the council finds them - no one noticed when they did minor things (things like Molly did in Proven Guilty) so they did those minor things and kept escalating until they were making waves that were large enough to be noticed. Kind of like no one noticing someone hurting small animals, then killing small animals, then following people, etc until he escalates to being a serial killer.
Richard