Knowing the area helps a lot (and I do) and I'd say it's private security as pickups are generally down on the beach/blvd level, not up in the hills... at least back in the day.
On another note, NOT knowing the area shouldn't have really been any concern and specifically pointing out that the whole world doesn't know where Mulholland Drive is or what PCH is... frankly not believable. These roads are very famous, but not knowing them only makes the situation OP describes "creepy" and knowing them actually makes it slightly less so as it's not a nasty area. But making a "thing" about saying you never heard of them is irrelevant, is in no way even called for by anything the OP said and only proves you:
A) certainly never watched CHiPs or any other LA based TV show exported to 100+ countries
probably never read a single LA based mystery novel, some of which are true classics
C) never watched a single LA based movie, some of which are also true classics
All are unbelievable if you ask me, but if you honestly fall under one of those excuses, I suggest you get a library card and try to stop fantasizing that most of the whole world doesn't know things about LA. Saying it's "typical American egoism" that makes people think otherwise is a total cop out. Book readers and movie watchers the world over DO know about LA through movies and books. Deal and try not to be such haters just cuz you can't get your California on.
OP: You are MUCH braver than I am. I still find it nearly impossible to cache alone in broad daylight in one of the nicest neighborhoods in North America. I could have never kept going to the point you did... props. A nice scary story for Halloween indeed, thanks for sharing.