UK Car Repair: battery power...., vauxhall vectra, earth connection

Question
Hi....
I am a little puzzled to my cars behavior lately!!!!
I have a 1996/97 vauxhall vectra 1.8 gls and everytime i go to start the car i have a flat battery, i went out on thursday of this week and the car was fine was also fine friday morning, then when i went to start it friday afternoon there was nothing ....battery totally flat.
I have changed the battery and it started first time,then this morning (saturday) it was totally dead again.
Just seems a little weird why a new battery would go flat over night and why they car starts to play up just like that without any signs or symptoms.
Nothing is being left on in the car e.g radio,lights it just the alarm/moblizer.
How you can help me overcome this problem,thanks in advance.

Answer
It sounds very much like a security system fault. To be sure, test the system for a battery draw by dicsconnecting the battery earth connection and putting a test lamp between the terminal and the cable, the light will be on if there is a draw. Now, remove the fuses one at a time and see what circuit causes the light to go out or dim.