What do you say to a child that returns something they stole from you?

You are not the parent. A child that is NOT yours stole from you, then returned it for whatever reason. I don't believe honesty should be punished. I believe empathy is good (but rarely instilled in a single interaction). So, I wouldn't be like: "wow you're a jerk how could you have done... show more You are not the parent. A child that is NOT yours stole from you, then returned it for whatever reason.

I don't believe honesty should be punished. I believe empathy is good (but rarely instilled in a single interaction).

So, I wouldn't be like: "wow you're a jerk how could you have done this" or "the only thing I feel about your returning this is pure anger."
I'm thinking something like: "losing this made me sad, but I'm happy you were honest. If you asked, I might've let you borrow/use/have it." I wouldn't bully small children about it. I don't want them walking away crying for (in the end) doing the right thing. That teaches kids to lie...

I absolutely DO NOT want to punish honesty, but I want them to have some sort of negative consequence, even if it's just feeling bad because they understand that what they did hurt someone in some way(s).
I also don't want to do abusive threatening similar to threatening to destroy kids' property.
What would I say to a 3-year old, a 12-year old, a 17-year old?

tl;dr
A child that is NOT yours stole from you, then returned it. What do you say to a 4, 12, and 17-year old to discourage them from stealing again *without discouraging honesty*?
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