A lost-and-found is where there's little or no business relationship and you didn't ask for the stuff.

If we have a formal relationship where you're holding on to my stuff or vice-versa as part of that relationship, you are generally required to return my stuff and I am required to return yours when the relationship ends. If I am working for you, and you have my tools at the office, you need to return them to me. If it's impractical for me to pick them up (e.g. I left them in an office in Alaska), that means you need to ship them to me at your cost. If a contractor has your data, and you don't renew the contract, they need to reasonably provide it back to you. If you are leaving a relationship and your ex has a bunch of stuff at your house, you need to return it. Etc.

That's very different from someone losing something on your property.

I suspect, in this case, if both sides had equal legal budgets, Google would lose a court case. Now, testing that is very impractical since legal budgets aren't equal, but from a pure justice perspective, that's how it works out.