I guess I did not think about offscreen or otherwise sneakily positioned form elements. I think the warning is the best way to handle this. I imagine it being something similar to an android permissions popup, where you can check off what should be "shared" (auto-filled) with the site before actually populating the form.

Then you'll love this concept: https://www.youtube.com/watch?v=3mk0RySeNsU A 1:18 "clickjacking" demo. (Link to video because I think this is legitimately one of those cases where a video can describe and demonstrate the problem faster than text.) You can google around for more info, or at least my YouTube is definitely popping up some other relevant videos.

Something like "Chrome has autofilled 12 fields" or something more userworldy would be good.

Or massive highlighting around each field