Looks like they’ve been filming in southern CA.
Restaurant Wars was filmed at a James Beard Foundation dinner somewhere “on the West Coast” on October 10th. AND it seems like it might be a full-blown All-Stars - there has been filming at Century City Mall, with sitings of Stephanie Cmar and Nini there. Someone else saw Eric A, Kevin from Season 16, Bryan Voltaggio (maybe judging?), Karen from season 13 and a few others at a farmers market near San Diego.
Another Reddit thread said "this checks out with Nini, Eric A, Kevin disappearing from social media from around september 15 after posting on a much more regular basis…i think we can say they are confirmed contestants. karen and bryan are probably judges
who else? a brief glance of season 15 contestants social media reveals Lee Anne Wong might be a contestant. she goes from posting every day to a final post “if you’ll need me, i’ll be at the beach” on sep 13"
Tom C. said filming wrapped in L.A. on the 21st and they were heading off for the undisclosed finale location.