Battery drain

I'm backkkkkk!
I was involved with a starter problem a few weeks ago and finally with all the expert help determined the battery was bad.
I purchased a new battery and since the Jeep has started on cue.
I haven't started it in a couple of weeks and when I tried it wouldn't start. The solenoid clicked continuously so I checked the voltage of the battery and it had drained to 11.5?
I disconnected the negative cable and applied my ammeter in series to measure current flow, as detailed in the attached diagram, and low and behold I'm getting a reading of 12 volts. This is after I recharged the battery for 48 hours.
I am under the impression that there should be minimum voltage with this setup?
The jeep doesn't have any circuits to radio, security device, etc. No lights are on so really I have no idea what and why the draining of the battery?
Would a kill switch solve the issue? I know it doesn't find the problem but if the battery is disconnected when not running there shouldn't be any drain on the battery?
Thanks in advance for your assistance.