"The boy is behind a big tree."
Translation:A fiú egy nagy fa mögött van.
My work-in-progress theory is that no article is more indefinite than indefinite article. It's like treating things uncountable. As if you could turn on the tap and fill some big-tree-material into your cup. "No matter the amount, whatever is in front of this dude, it's of big-tree-material".
Some further evidence of no article being the most indefinite way of articulation: in a text, it's very rare (virtually nonexistent) to later reference something that has only be mentioned in singular and without an article. While indefinite article creates a reference you can go back to, using definite article - no article doesn't really create a reference like this.