This just means: What did you think we found?

¿Does the должны были in Elena's translation actually mean MUST/SHOULD HAVE, or does it express an ASSUMPTION on the part of the speaker, like Он, должно быть, механик. He must be a mechanic.

As far as I understood that, they had already found something. And this is the way someone tired to make someone else to guess what it was.

Actually, they had found something and they were curious as to what exactly the listener thought that was. It expresses confusion/curiosity more than anything.

I posted the translation as it's published in Russian (from "Tom Sawyer"). It expresses curiosity.

That's it! 🙏

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