Batteries self-discharge, even when disconnected. 5% is probably not enough to last months, especially if the car is exposed to high temperatures.
I bought a new laptop, so closed down my old Macbook, and stuck it away.
A few months later, I found I wanted to grab file from the old macbook, and noticed it wouldn't run from battery. It'll work fine plugged in, but the battery is registered as full discharged, and the Apple page says "Buy a new battery".
The other thing to consider is that as the voltage drops, so does the leakage current.
Really, this whole thing could be solved by putting a tiny solar panel on the back dash.