Alarm goes off for no reason 20-30 minutes after i lock doors

Not sure about yours, but many will reset by: turn key to "on" and wait 15 minutes or so; shut off, then try to start.

IT is probably just letting you know that it has received the lock signal and has secured the vehicle. Horn beps or clearence lights flask

are you irish ?? thats your alarm light warning and its meant to blink to warn any would be tea leaves that this vehicle is alarmed with a chipped key ,actually you better off leaving it unlocked as then the windows will not get broke by a junkie looking to see if you have any emergency cash in the glove box

Had same problem not to long ago~~~~ I have to check all 4 doors to be sure they are locked mainly the back 2 and then I went around to all 3 door locks & locked and unlocked a few times each... it was on passenger side when my light finally was deactivated... hope this helps you.. I haven't had any problems since ;0)

You can disengage alarm with a remote if you have one otherwise, close bonnet, all doors and lock the car by turning key completely to the left and right few times and do the same to unlock. This will dead lock and unlock the car. When you hookup the battery make sure you connect red wire first and then the black one.

Now im not sure since the manual says your model has the same alarm system as the 328i and on the 328i you can remove the module to disable by doing the following. you can unplug the control module. It is located under the glove box. Climb under the dash on the passenger side and remove a few screws on the trim panel under the glove box. Theres a black control module with a green connector. There should be a lock tab on the connector. press the lock tab and pull out the connector. The alarm should now be disabled. But if it has the special transponder key this may not work.
But if the only reason you want to disable it is because of the motion sensor setting of the alarm for no reason there is a solution for this. To disable the motion sensor, hit the lock button to arm, then hit lock button again within four seconds to disable motion sensor. Red light on rearview mirror should flash four times, then pause, then start flashing on regular intervals. this is most likely the reason if your having the issue where the alarm goes off every 20 minutes.
If you want to completely remove it below is a wiring diagram taken from the manual for you alarm system.
this is from the manual its the alarm wiring diagram.
Battery Constant 12v Positive Wire ( ): Red/White or RedBattery Constant 12v Positive Wire Location: Ignition Switch Harness
Starter Positive Wire ( ): Black/Yellow or Black
Starter Positive Wire Location: Ignition Switch Harness
Ignition Positive Wire ( ): Green
Ignition Positive Wire Location: Ignition Switch Harness
Second Ignition Positive Wire ( ): Purple/White or Purple
Second Ignition Positive Wire Location: Ignition Switch Harness
Parking Light Positive Wire ( ): Gray/Purple and Gray/Yellow. Use Both Wires. This vehicle has a dual parking light system that will require 2 extra relays.
Parking Light Positive Wire Location: At Headlight Switch
Power Door Lock Positive Wire ( ): White/Red
Power Door Lock Positive Wire Location: At Factory Alarm Plug. The alarm plug is a black 12 pin plug located just behind the glove box.
Power Door Unlock Positive Wire ( ): Blue/Red
Power Door Unlock Positive Wire Location: At Factory Alarm Plug. The alarm plug is a black 12 pin plug located just behind the glove box.
Door Trigger Negative Wire (-): Brown/Gray
Door Trigger Negative Wire Location: At Factory Alarm Plug. The alarm plug is a black 12 pin plug located just behind the glove box.
Domelight Supervision Wire: Turns On With Unlock
Domelight Supervision Wire Location: N/A
Horn Negative Wire (-): Brown/Red or Brown/Yellow
Horn Negative Wire Location: At Steering Column Harness
Tachometer Wire Negative Wire (-): Black
Tachometer Wire Negative Wire Location: At Instrument Cluster
Brake Light Positive Wire ( ): Blue/Red or Green/Red
Brake Light Positive Wire Location: At Switch Above Brake Pedal
Factory Alarm Disarm Wire (-): Disarms With Factory Remote Only. Factory Alarm Disarm Wire Location: N/A
Factory Anti-Theft System: Transponder Anti-Theft System, Requires Bypass And Extra Ignition Key. The 1997 model year vehicle may require a bypass module and an extra ignition key, if vehicle is equipped with a special key from BMW.
Factory Anti-Theft System Location: Transponder At Ignition Switch Tumbler
if you require further assistance please let me know and i will help assist you from there. Thanks, Midwest-Tek

red light? this is a factory alarm?? Is GM on the key fob? factory alarms dont have "motion sensors" and very few aftermarket alarms do. roll down the window and set the alarm, now open the door and see if it lights off.

sounds like the hood alarm switch is not making proper contact with the hood, the alarm will not set properly, check the hood alignment

you may have a broke wire in the door harness

There is a security feature that will allow to go for a short distance but will stop. It is basically the same as the alarm, when you lock your doors, and then shut the door as you exit, there is a red security light that will blink in the dash. If you manually unlock and open any other door besides the driver door, the alarm will go off. To disable the alarm you have to cycle the key in the driver door. The same goes for the security feature. Cycling the key in the driver door lock manually, should disable the security feature until the doors are locked again. There may be a problem with the remote if you use it instead of you key to unlock your door.