2000 Mercedes Benz C200 alarm

hi,today my car alarm siren goes off when i lock it with the remote,it is fine if i use the key wich is inside the remote fob to lock it,i have checked that all the doors are closed and the bonnet and boot.i have replaced the batteries in the remote also,the alarm is still going off,please do u have any information on what to do?
thanks,andrew.