Thanks for the update! The security guards in the "shack" will tell you to watch out for the Airport Operations cars that come by the fence. If they The Op guys) see you they will ask you to leave.
The smart thing to do, not just at the Northwest Cargo lot at LAX, but anywhere that is "sketchy," is to get out of your car, snap your pictures, and get back in your car. Also, don't park right up on the fenceline.
Nothing sucks more as a spotter then to be in a location, peacefully, and have some jerk roll up in a rental car, park the nose of it right against the fence, and start standing on the hood or roof with a camera.
Just use common sense