Merc A-class battery drain

My neighbour has a 1999 1.4 A-class four door hatchback,recently it has been it will not start if left parked for a week,she has had a new HD MB battery fitted,the charging checked and all the usual things but to no avail,she asked me to have a look after her £200 trip to the garage. I to checked the usual interior lights,glove box etc but can't find anything remaining on,removing the lead from the battery and putting the test lamp between the cable and the post the light shows and stays showing,so something is using power,I know the alarm and CD memory and clock would use a minute amount but not enough the drain the battery fully in a week so the locks wont come off,jump start the car and it will start on its own after 5 minutes,any help would be appreciated.
Thanks in advance
Jim