honking horn dead battery

Why does the horn honk when the battery on my 96 trek jetta is low. every time the battery gets low and dosent have enough power to turn the starter the horn will start honking like an alarm. After charging the battery when I hook up the battery the horn goes off. Any information on this?

same thing happened to me today, i just rolled down my window and put my key in the door and turned it, turned off imediately.

also i like how the Jetta does that, but also you might think it was something else, i'm having a mechanic tune a full engine overhaul tomorrow..

Der Talon