Last November, Google said they are experimenting with the mobile-first index but recently told us it isn't launching any time soon.
But now some are seeing the mobile-first index in the Google desktop search results.
A WebmasterWorld member said when he searches for [Ikea USA], [Macys] or [Zalando] amongst others, he sees the M dot URL come up in the search results, instead of the WWW.
Personally, I am seeing www.ikea.com/us/en/ and not m.ikea.com/us/en/ but this searcher is seeing m.ikea.com/us/en/ for the Ikea search.
Of course, it wouldn't surprise me if Google is testing this with more users. Have you seen the Google mobile-first index in the wild yet? I have not.
Forum discussion at WebmasterWorld.
Update: This is likely not a mobile-first index test but rather related to Google Bug Shows Desktop URLs In Mobile Results.