Blowing your air horn when leaving. What do you want man?

I will take a wild guess. It is a carry over to days gone by.

When I as 10 or 11 years old,(1954/1955) people used to "stop by for visit". And when they departed from your home they tooted their horns very, very briefly, and waved goodbyes.

It was a common practice at the time. And I have no idea how it started.

But I just think it is mostly a lingering tradition from the olden days.