I have an aging, 2001 high-mileage A6 2.7T. On occasion when I lock the car with the remote and it sits in the hot sun the alarm goes off. Similar thing happens if I unlock the car with the remote and delay switching the car on ... but this time the doors lock and then the alarm goes off.
Thoughts?

 

The first issue could be the convenience module dying on you (heat does make electronics go crazy) that controls the alarm system.

In regards to the 2nd issue I'm pretty confident it is your door lock sensor. By unlocking the car and getting in, your car normally would know you stepped inside - but because the door lock sensor died it doesn't know you opened the drivers door and it locks the car again after 30 seconds (arming the alarm while doing so). You can easily tell by opening the door while the car is running and see if the dash shows you that the drivers door is open... or turn on your lights and open the door (while not running) - it should beep at your loudly if the sensor works.

 

Thanks Sheldon, makes sense. I'll check into both of those possible solutions.