Something is draining my battery

[mr.cobra]

Ey everyone I have a 03 cobra, just put a new battery in it and if the car sits more then 4-5 days the battery is COMPLETELY DEAD! I know it's not a crap battery because I have a snap on battery tester and after charging it fully it shows it's fully charged, but as soon as it sits for 4/5 days It won't even turn over, any suggestions on what's draining it or how to trace the problem?
FYI everything is complete stock including radio and there's no lights left on or nothing plugged in
Thanks

[nickf1989]

Does anything happen when you turn the key? If its COMPLETELY dead, test your alternator

[mr.cobra]

Does anything happen when you turn the key? If its COMPLETELY dead, test your alternator

I don't think it's my alternator because if I drive it daily it's fine or drive it for hours it's fine, but when the car sits for 4-5 days without be driven the battery is completely dead and needs to be charged again