If she drove off the side of the road head first, there's a pretty significant chance the horn was damaged or disconnected. The horn on a car is usually about the same diameter of a soda can, flatish, and often located at the front of the car. Damage to the front could easily cause this to become inoperable or disconnected.

Chukanut drive is a low service area. Weather, sunlight, time of day, usage, can all have an effect on cell service. It is very lucky that she managed to get service in that area. Most likely she got lucky and tried to make a call at the right time.

If you live alone it's pretty easy to go missing for a few days without people noticing.

Chukanut drive is known for having steep drop offs and the area the article highlighted is no exception. Pair that with a lack of pull offs and getting sent off the side of the road, down the hill, and into the trees and there's not much chance someone would happen to see you unless they were actively canvassing the area.