Battery drain/connection issue?

I have the car listed above with low mileage and it’s having some major battery issues. Every once in a while, in random spurts, it’ll start rapidly losing charge and I have to plug it in. I have taken it to a mechanic who said it’s a battery drain issue that happens on older cars and was unable to diagnose the cause. All summer it was completely fine without any issues. Recently however, the battery has been draining again, even to the degree of ruining the battery. The issue I am currently having is an odd one. Today, I went to start my car when I found out that it was dead. I got a friend to help me out and we hooked up jumper cables to the car and let his car run a bit to try and charge the presumably dead battery. After a bit I noticed the door lights of the car starting to come on. I turned the ignition in the car to try and get it to turn over. As I turned the key, it seemed to try and turn over, but as I did this, the dash lights flickered along with the engine as is sputtered. I’ve never really had too much experience jumping cars, so I don’t know if this is normal. Anyway, I eventually got the cars started by applying some gas. I let it run a bit as I say in the driver's seat, but when I opened the door, the whole thing quit, and I could get it to turn over again. I took the battery out of the car and brought it home only to find out that the battery was at 95 percent. I don’t know if this is a short or something of that nature? The cables were securely connected to the battery so that was not the issue. What do you guys think?