The Alienoid

It's the year 3327. You are on a spaceship on its way to prepare a new discovered planet for colonization. Since it is a long trip, the crew is in cryo stasis. You are part of the maintenance crew. Every year you and your team members switch places, only one is awake, the rest sleeps. Due to some unexplained events you woke up sooner than you should. Protocol is that the one ending his shift wakes up the next one before he goes to sleep, but when you wake up, there is no one there. But you here strange noises, like someone is moving through the ship and interacting with it. What happened?