Is it safe to say that if I've never seen an AdWords conversion from a non-google.com domain, I should disable search partners? Do they always come in as non-google.com domains?
The answer seems to be no:
On other search sites that are part of the Search Network ("search partners"): Ads might appear alongside or above search results, or on other relevant search pages, when someone searches on Search Network sites. Our global Search Network includes Google sites such as Google Maps, Google Shopping, and Google Groups.