I checked key fob battery. It was probably OK (measured 3.0 volts) but I replaced it anyway (new one 3.3 volts) since it is a bit of pain to access and I don't know when it was last replaced. That didn't fix the "horn chirp" issue. In looking at vehicle further I noticed a small, obviously aftermarket red LED mounted to left of steering column in dash. It flashes rapidly when horn chirps. So I figure the "horn chirp" is due to a dealer-installed aftermarket alarm system which was put in by original owner of vehicle and current owner doesn't have documentation on. I plan to look up into dash to see if I can locate the alarm control module to find out brand/model to research further what the "horn chirp" is indicating. As an example I did find an owner's manual posted online for a Karr Security Systems device (which looks to be a common type of dealer-installed system) and see it uses 7 different horn chirp patterns to indicate different conditions. So once I figure out what system is in this vehicle I hopefully will be able to locate an applicable owner's manual posted online.