The intake manifold doubles as the fuel rail. The fuel travels under pressure (60-100psi, or simular) down each side of the manifold, where it then branches off and feeds the injectors on that bank. There should never be gas inside the intake manifold, only the cylinders. It should be sealed, and pressure tight, right up until the injector itself, which only opens on command from the computer. The only thing that is suppossed to be inside the intake manifold is air. The only part I am not sure about is whether the injector sprays gas directly inside the chamber, or just ahead of the intake valve.
Maybe there is a crack inside the manifold, and gas is flowing in. Has the manifold ever been extrude honed, ported, or the like?
I'd say its unsafe to drive the car, because if that gas got heated by the engine, turned into a vapor, you could have a fire, or worse, an explosion. Park it, yank the manifold, and check for leaks?