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"Is this unethical if everything is done locally and no data is stored or resold?"

If it is hidden (no clear warning) and there's no way to opt-out, then yes, it is unethical imho.

Nothing unethical, just the fact that it is hidden may cause some outrage.

Then what would be the purpose of collecting that data in the first place?

Yes. It is unethical regardless of how it is done.