Turning headlights on honks horn

My son has a 1998 Ford Mustang with an un-known type aftermarket alarm.  It uses the stock Ford alarm fob is that halps.  He just installed HID headlights in it, and interestingly, when you turn on the headlights, the horn makes a very quick "beep" sound.  This happens whether the engine is off or running.  I assume it has something to due with mementary current draw then turning the headlights on.  Is there a choke or supressor that might be able to be used in the power to the alarm to stop this?  Is there any way of figuring out the brand of alarm (was installed when he got the car), such as searching for a control box, etc.?  Thanks, TIm

Did you have this horn problem BEFORE you installed the HID's?

No problems before the HID install when turning on or off accessories.  My son did say that if the car battery gets quite low (from the car sitting unused for a while) the horn may honk when starting the car.  This kind of lead me to believe that a voltage drop would trigger the alarm to sound momentarily.  The HID's are Hella (very good brand) and were installed as per the instructions.  Across the battery terminals with all electric items off and the engine off, the voltage read about 12.5 VDC.  When turning on the HIDs it dropped to about 11.50 VDC if this helps.