There is a popular philosophy that can be summed up like this: Look out for #1. This sounds more like an advertisement for natural selection than it does the basis for an ethical system. The biblical writers take a different tack.
The writer of Deuteronomy records God's admonition that rather than selfishly looking after our own interests, we are to care for our brothers and sisters. Our brother's sheep that runs away is our responsibility to return, even if no one's looking. We are not to hide behind plausible deniability, but to do the right thing.
It goes on to say if we find an ox running loose and don't know who owns it, we are to take it home, feed and care for it until we find out who it belongs to, and then return it. Although it might be easier to ignore the animal, or more advantageous to keep it as our own, that's not the way for one who claims a relationship with God.
I can't remember the last time I came upon a wandering sheep or ox. So how does this apply to a 21st century inhabitant of a major metropolitan area? Jesus put it this way: Do to others what you would have them do to you (Matthew 7:12).
Seen any wandering oxen lately?