No key in the vehicle

[GlobalHK]

For the previous months, my car S8 D3 kept beeping during driving and showing "No key in the vehicle." The situation is getting worse now, most of the time I can't open the door with the keyless function at once and have to try 3-4 times. Also can't start the car because of "no key".
I have changed the battery and also tried both keys and the situation still unchanged. so I pretty sure the problem come from the sensor in the car.
Does anyone know where does it locate? and how to solve the headache?
Thanks.