Originally Posted by
pharasz
So what did I accomplish?
Nothing. You may have gone over the line.
Passing
Stay a safe distance behind the vehicle you want to pass. The closer you get to the vehicle you want to pass, the less you can see ahead. This is especially true when passing trucks, trailers, and other large vehicles.
Before you pull out to pass, check your blind spots and make sure that you have plenty of time and room to pass.
On a two-lane road, tap your horn, or at night blink your headlights to let the other driver know you are passing.
http://www.lowestpricetrafficschool..../driver/en/3/9
Passing
Stay a safe distance behind the vehicle you want to pass. The closer you get to the vehicle you want to pass, the less you can see ahead. This is especially true when passing trucks, trailers, and other large vehicles.
Before you pull out to pass, check your blind spots and make sure that you have plenty of time and room to pass.
On a two-lane road, tap your horn, or at night blink your headlights to let the other driver know you are passing.
http://www.speedingticketonline.com/chapter3.aspx
There are several other sites that cite this same material. The driver could have just said he was warning you before passing and you wouldn't have anything to back your position.
On the other hand, depending on the situation, a horn from behind is a warning to me that the motorist sees a potentially hazardous condition coming up and I should take the center of the lane. If the driver is just warning to pass, then they will be changing lanes anyway, so me being a couple feet further to the left won't make any difference to them.