Unannounced to the marines, GF had snuck aboard the ship but had been drugged by some scientists and was put away to be saved for a Compound back on Earth.
--On the Bridge--
Weyland Android, "Pump drugs into the vents of the crew. We are gonna have a little experiment on earth."
"Sir Yes Sir"
The pilot pumped in the gas and the whole crew went unconscious and the ship raced towards the Earth, towards the compound.