It opens.if the car is sitting parked an i close the boot after a few seconds you can hear the boot clicking away it opens itself
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most likely this is a simple faulty boot lid lock (or possibly faulty boot release button or wiring short) - the offending part will simply need to be replaced.
one thing you can try as a temporary fix would be to secure the boot seperately:
for E46 sytle 3 series: turn the key in the boot lit 90 degrees clockwise then remove the key with the keyhole in the horizontal position
for E90 style 3 series: flip the switch in the glove box to permanently lock the boot
here are pictures to help you identify which style you have:
e46: http://upload.wikimedia.org/wikipedia/commons/c/cb/BMW_3er_%28E46,_1998%E2%80%932001%29_front_MJ.JPGPlease let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your vehicle.