1--A new battery probably was not needed. A recharge of that battery and finding and fixing the problem is the solution. My 2016 often sat a week, with the original battery, and no problem.
2--Some CX-9s with the toe operated lift gate opener had the wayward electrical drain through that toe operating mechanism. That's the only known problem of this sort I've heard about, and it can be easily fixed. I not the toe switch, then something else as described in the posting above. A trickle charger (the modern name is a battery maintainer) is a band-aid, not a fix.