I had this a while back every 2 days it was flat, purchased a new battery and the same thing happened was advised by people on here to check alternator, glove box and the light in the boot was not staying on, which i done.

Then a friend told me about his Escort Cosworth doing the same thing and he called a auto-electrician out and it turned out the battery from his tracker had corroded and somehow managed to stick to the floor pan causing a constant drain.... If you have a tracker check this failing that its get a multimeter connect it up and then remove 1 fuse at a time to see what circuit the drain is on then further investigate that circuit, good luck and keep us informed