English: “Straying” — excluding those that are [already] lost [in the sense of having been abandoned].
On the daf / in context: The term “niddachim” is taken to refer to animals that have wandered off but are still recoverable, as opposed to those that have become completely ownerless.
Type of Learning
Limiting derash on the word “niddachim”.
What Is Learned
The biblical obligation of returning applies to animals that have strayed but are still identifiable as belonging to someone, not to property that has become ownerless.