Switching the ignition on/off twice with doors shut then getting out and immediately locking using the remote will disable the interior sensors and should beep 3 times, if the fault is with another part of the alarm then it will keep going off.
If you have locked the car manually from the button on the dash then got out the passenger door and locked the car using the key then the alarm will not have been armed, the led by the gear stick should not be flashing.
I had a problem with my alarm not working when i picked my car up and found the alarm had been disconnected under the dash because there was an intermmitent problem with the alarm going off, if you look under the dash in the passenger footwell you should see a wiring harness covered in insulation tape, there maybe 1 or 2 single red/white wires with connecting plugs poking out of this harness to identify it, this harness will be plugged into a 6 pin black multiplug with blue sides and is clipped up to a bracket, just disconnect that multiplug and that will disable the alarm.