According to a friend, both Chevrolet and Toyota have proving grounds in the Phoenix metro area. I'm not sure which brand these would be, so I'll leave it to those of you who are more "in the know" of up-and-coming vehicle models. These wouldn't be anything to do with GM'* Zeta platform, would they?
Since I didn't get a good shot of the grille through the cloth mesh cover, let me describe it. It had chrome horizontal bars that were fairly thick, though in visiting both the Toyota and Chevrolet websites I see models from each make that feature thick horizontal grille bars. If I had to lean one way or the other, I'd say the grille on the test car was more like a Toyota than Chevy.
Do the lines of that car roughly resemble a Toyota Avalon?