alarm/car battery problem

Hi folks my mate has a p reg disco his problem with it is when he locks it with remote fob it locks/alarms but when indicators flash 3 times it also beeps (its not done this before) when he comes back to it it will not start due to battery being flat , starts ok when booster leads used ,garage looked at it and it was ok (no beeping or flat battery) for a while now doing it again (new battery fitted) its now stuck on drive with a flat battery any thoughts would be wellcome
thanks
posted in other part of forum but no replies so stuck it in here thaks for any info