Battery Voltage Loss With Key Out

I am pretty good with automobile electrics. Not an expert like many others on this forum, but I get by. It sounds to me like @YO7_A66 is trying to track down an open circuit that is draining the battery. If you take the negative cable off the post and check for current flow between the post and the cable, there should be nothing unless a radio with a constant drain for the station memory has been installed. If there is a drain somewhere, there will be a current flow between the post and the cable. Then I would pull the fuses out one at a time until the current flow stopped. Then, at least, you will know which circuit to examine.
Also, when @Dana67Dart asked if your car had a trunk light, that is a great thought. If the trunk or glove box light switch is bad and the light is on all the time, that will drain a battery. in a completely dark garage, you can often see a little light if one of those bulbs is on. Another way is to open the trunk and immediately touching the bulb. If it is hot, that is likely the problem. A bulb won't get hot in the second it would take to touch it right after opening the trunk lid. Have fun.