Mine does the 3 beeps on unlocking if the internal sensors are turned off, My alarm is very prone to going off randomly so if the car is somewhere safe at night i turn off the internal sensors so the alarm doesn't go off. Note: you turn the sensors off by turning the ignition so the dash lights up twice and then locking the car within 20 seconds.
As mentioned the car beeps a certain number of times when you unlock it if the alarm has gone off, apparently the number of beeps when you unlock it corresponds to a fault which is also apparently in the handbook, I don't have my handbook so I can't check the beeps!
Mine does occasionally beep when I lock it but as mentioned I've heard its to do with locking the car too quickly after shutting the doors (its a matter of seconds), also if you unlock my car and open the doors very quickly it sometimes beeps.
So essentially I haven't helped i've just said mine does it too :dapprove: