Key-On Warning System
WARNING: This page is about a different car, the 1990 Acura Legend. However, it is still accessible from the selected car via links, so may be relevant.
When the ignition key is not removed, the key-on beeper in the integrated control unit senses ground through the closed ignition key switch. When you open the drivers door, the beeper circuit senses ground through the closed door switch. With ground at the A2 and A1 terminals, the beeper sounds.