Maybe was taken apart to build another racer/prototype? That happens from time to time.
I may (actually DO) sound bad when I say this but Group B cars really make me kinda-sorta wish we didn't care about safety so much x_x . My father and I were talking about this recently, how in F1 the specter of death is almost completely gone. Sure you can still get seriously injured in all forms of racing, but it just feels like we've lost something.
Of course runaway budgets also have a lot to do with what we have today in Indy, Rally, F1, etc..... but some of the "fun" went missing (admittedly along with untimely deaths) when we took away the 1500 Hp IMSA racers and insane Groub B cars.
http://www.speedhunters.com/2013/01/racings-the-last-golden-era-the-1980s/