Richmond, VA 2.7T

I have been experiencing some strange behavior with the central locking system in my 2000 A6 2.7T. Once or twice a month, when I lock the car using the remote, the alarm system does not activate (in other words, the red LEDs on the doors do not blink as usual). If I unlock the car and then immediately lock it again (without opening any of the doors), the alarm system acts normally and the lights blink.

Upon scanning the control unit with VAG-COM (V303.1), I found the following DTCs:

01374 - Alarm triggered by Terminal 15
35-00 - -
01370 - Alarm triggered by Interior Monitoring
35-00 - -
01368 - Alarm triggered by Luggage Compartment Switch
35-00 - -
00955 - Key 1
09-10 - Adaptation Limit Surpassed - Intermittent

Probably only the last one is applicable, but I am not sure. Also, I have been unable to find any information about the meaning of "Terminal 15". Any thoughts about what the problem might be?
Thanks for your advice.