You wouldn't expect ghosts to show up in a straight action/stealth game like Hitman. The series is known for its gritty realism, and besides, the whole point is that you can kill pretty much anyone you see in contrived ways, so why would they even bother to include the one type of person that's completely immune to being shot at?

The "Traditions of the Trade" mission in Hitman: Contracts takes place in a hotel where you have to find a device and kill some people. At one point during the mission you come across this door

The door is closed, but if you pick the lock while nobody is looking, you can go into a hidden area of the hotel. Once you're in there, if you hurry up you can catch a glimpse of something going through a wall. Take too long to get there and you'll miss it.

At this point you have two options: You can turn around, pretend you didn't see that and go on with the rest of the game -- or you can keep going and find out what's ahead. If you do, you'll run into a cop who tells you there's been a "nasty accident" in one of the rooms. There's blood all over the place, a dead body on the floor and a knife on the wall.

And this is where it gets creepy: Walk into the bathroom and you'll see a bloody smudge on the wall and a bathtub filled with even more blood. That's all there appears to be in there. However, if you happen to look in the mirror ...

... a ghost flickers into existence. You can actually see him in the bathroom of another room while a dude showers, so apparently this is a spirit of the voyeuristic kind. In this scene he's probably just waiting there, hoping he'll get to see you take a crap.

The ghost has no relation to the plot whatsoever. It's not an achievement or anything like that, and you can complete the whole game and still get the highest rating while being blissfully unaware of its existence. The bloody hotel room itself is also kinda creepy: that's not the most grisly murder scene you'll see in Hitman, but the difference is that in every other case, you always know the exact reason behind the tragedy.