The disguise was for transport. There was very little chance that it would be discovered once underground and even if it were, it would not compromise the owner as it would be located far away in a place like a wood or a public park.
While it's true that a dumped car battery would at least raise interest or even suspicion, a buried one is significantly harder to stumble upon, however suspicion is still optional. Who is going to open a potentially highly acidic/hazardous car battery anyway? Something "rattled" inside, which suggested it was dry, and what ever rattles inside a car battery, everything is supposed to be fixed there, right? Besides, an ancient Varta battery - put that in a museum! There - the cover's blown.
Back in the days no one cared about dumped car parts. But nowadays just about anything raises instant suspicion if anything out of the ordinary lies about on the ground.
Are we sure it was buried intentionally, and not moved from its original place and eventually thrown away (presumably because the concealment worked as intended and it looked like a junk old battery) to where it ended up now?
I can't edit my other comment, but I did think that it accomplished its mission.
> ensuring that anyone who stumbled across it would be suspicious.
Or the person who was meant to retrieve it would know what they found. "It's inside the car battery buried in ..." is a pretty easy instruction to follow.
The instruction can also be hidden in plain sight – a car battery is ordinary enough that no one is going to be suspicious if you write it down on a piece of paper. You can even hide the location in order or invoice number pretty easily.
If someone found it, they may be suspicious. If I found one my first thought would be that is just some junk someone threw out (you can't put car batteries in normal trash where I live). It is fairly easy to notice if dirt has been recently disturbed, even from afar (which his good if you're worried about getting caught).
TL;DR burying something in a car battery is actually pretty clever.
