When Mechanical Key Is Used

WARNING: This page is about a different car, the 2009 Nissan Sentra. However, it is still accessible from the selected car via links, so may be relevant.

With the key inserted into the key switch, and the ignition switch in the LOCK or ACC position, when driver's door is opened, the warning chime will sound.

BCM detects key inserted into the ignition switch, and sends key warning signal to combination meter with CAN communication line.
When combination meter receives key warning signal, it sounds warning chime.

Fig 1: Key Warning Signal Diagram (With Intelligent Key)
Courtesy of NISSAN MOTOR CO., U.S.A.