According to a story in the Detroit News (here), GM is considering moving many employees out of downtown Detroit's only modern high-rise, the Renaissance Center, to fill in the empty offices in Warren, where many of the designers and engineers sit. GM tried to sell the RenCen and then rent it back recently, but couldn't find a buyer. And Warren's mayor recently campaigned to have GM move its headquarters entirely back to Warren.
It's bad for Detroit, which is already teetering on insolvency, but GM has to look out for GM. If Warren is better for cash flow, then Warren should win.