I have a Renault Scenic Megane 1.6, the battery drains every 3/4 weeks? Can you help me to find out why?

I hate to say it but it sounds like a job for an auto electrician as opposed to a garage. The only thing that comes to mind is the failed door lock motor which may have been shorting and taking voltage as it died. If the flat battery still happens it can't be this of course. As said, bite the bullet and seek out a specialist. They have all the kit and knowledge to trace electrical faults