2000 Rx-300 Horn Blows On And Off After Shutting Off Motor

[lowrx300]

When I turn off the rx it starts blowing on and off the horn and flashes the lights. If I shut the rx off after I have opened the drivers door it does not have a problem. I know it has to be the factory alarm but not sure how to fix problem. Any input would be greatly appreciated. Also this problem begins every time the battery must be disconnected. The problem eventually stops but don't known the reason it does.

[1990LS400]

Yes, disconnecting the battery sets off the alarm. The attached two pages for your owners manual explains how your alarms works and how to reset it.