I have never reloaded a game because I could not find an ennemy.
The thing is, you need to look in places you would not think about finding an ennemy. Like a hole in the deck of a ship trapping a NightOne, or an ennemy which spawned behind walls... That's when it is time to prime grenades and go Alien hunting.
Don't forget, Aliens usually always spawn at or around the same place...