I do think your battery is on its way south. I think mine is too but keeping on the tender will prolong its life a good bit since mine is 5 years and 7 months old.
I agree that the problem is they didn't load test the battery. That is the only way to determine if it will be OK or is getting weak.
I am guessing we will see a post from you in the next few months about battery replacement too.
In regards to how do I keep mine charged I honestly leave the tender in the car on the pass side where the jump posts are. I hook up the jump clips to the posts and lay the tender in that general area. When I can I pop the hook and just run an extension cord to the tender. I have also on my M3 just run the clips to the hood and pop the hood open and plug it in too but leaving it in the car engine bay has not caused any issues with it moving around or getting over heated from engine heat in 10 years so I am guessing it is fine in that location.
I experiemented with the best way for me to not spend time doing this and leaving it in the engine bay serves me the best. It really only takes a minute or less to do this and the only down side is to not forget you plugged it in. Yes I have forgotton a couple times and pulled the extension cord loose but other than that it works well.
Smart chargers regardless of which one you pick should keep your battery up.
I will now lock my car in the garage since I was not aware until now of the car doing a pulsing of the system which can only drag your battery down. Why BMW would do this I have no idea but it seems excessive as does battery registration...
My E93 is in Ohio until the end of this month of I would have taken pics of where I store it and the clips too which would give you an idea of where it sits patiently waiting to be used again...