The Torah presents and ethical ideal.
If you find something that is lost you return it.
The Mishna presents a more practical ideal for social order and peace.
This simple law of 'if found return it' is explained with more practical applications. There are lists of things that do not apply to the rule such as fruit or even a coin that may have been scattered. And Mishna continues to give rules for how to proclaim that an item was found to help find the owner.
Then the Gemara goes another step and instructs the finder to think if the person who lost the item would 'expect' to recover the item(s).
Mishna & Gemara on Found Items--- Then there is the Environmental Interpretation:
We are merely a caretaker of the world and everything belongs to God. We should take care of things and then return it.
Interesting thought.