It will beat my conscience really hard to knowingly keep something that doesn't belong to me. Regardless how little, I would try my best to return it.
I wouldn't like to be cheated it I were on the other side, so I wouldn't do the same.
It's a really kind thing you did for that woman. It's not much of a big deal, really, because that's how things should be done, but the world we live isn't normal.
That's the thing about having a good conscience, it won't let you do things that you aren't meant to do.
I usually don't see a big deal in doing things like that but it's a big deal in the world because things like this barely happen anymore. People just want to take advantage of others, reap where they didn't sow and others.