Land Rover Repair: Disco 2 (2000) Electrical/ Alt issues, dead as a doornail, ammeter

Question
So with a relatively new battery (3 months) suddenly drained out I decided to replace it and figured it probably went bad. Got the new one in and things ran fine for a few hours parked the car for the night and in the morning the battery was dead as a doornail. Went and got the Alternator tested and it was going bad so replaced it with a new bosch one. I made sure it was set in properly double checked the wires and ground and put on a spare battery. This worked fine and after it ran for about 15 min I again went to bed. The next morning the car struggled to start and once it ran for a few seconds I got those wonderful M and S lights flashing. So tested the battery and it was just drained and got the Alt tested and that was fine so am I just going to ground somewhere i shouldn't be or is there something else that could be the cause?

Answer
It sounds like something is remaining on and draining the battery.  You can hook an ammeter in series with the battery power leads and then try unhooking the accessories one by one till the drain vanishes.  Once you find the circuit, you need to isolate that to the actual device and decide what to do.