Probably going to need a full ISTA diagnostic

My 2015 435i died last week Wednesday on the first day temperatures went below 0. I jumped it and was getting a battery discharge error so I replaced the battery with a new one and the error never came back. The car then still died overnight. I brought it to a shop and we are going into day 3 of trying to find the drain. They know it’s draining but the mechanic said he’s getting to the end of what he’s able to do. So I’m going to need to go to a dealership so they can find it. I just don’t get it, did I just get unlucky and something went out or does anyone have ANY advice?