Any time you kill a guard and shoot his eyes, they normally will not get bloody. However, there is a guard in the Plant chapter where this does not happen. After you have defeated the Harrier you will proceed to a path on the side of one Shell. You will find a guard on the roof that urinates off of it. Use your silenced AK to shoot him down. This is very hard, but you can have a chance to shoot his head when he speaks about his urge to urinate. If you miss the first time, shoot the wall under him. He will walk to the noise, then return and start talking again, giving you another chance. When he falls, approach his corpse and shoot his eyes, which still will be open. They will have blood in them.