Task Force Spartan
Principles of Stealth
While these techniques are aimed at operating in a low-light environment, the general principles can be applied to daylight operations as well by careful players. Remember that all forces are not created equal - while you may find great success through stealth when night vision and thermal devices are not present, the presence of either of those can dramatically shift the equation. Night Vision still permits stealth to occur, just with more caution in employing it.
Stick to shadows and minimize movement
The value of shadows should be obvious. When you stop, place yourself in brush and shadows that can distort or mask your shape. At night, people are looking for familiar shapes or outlines, and are able to notice high-contrast as well as easily see movement. Be aware of your backdrop - being in the shadow of a tree will do little if there's a bright white wall behind you from the enemy's perspective, silhouetting you.
Take your time when moving
You don't want to rush in the dark - it's too easy to miss people and end up getting shot in the back in the process. The same general rules of daytime tactical movement apply to the night - there's just more emphasis on avoiding contrast, staying in shadows, and trying to stop in locations that mask your visibility. Move from one position of concealment to another, observe, wait, listen, then move again when the coast seems clear. Hiding in bushes, trees, and thick brush makes you far more difficult to notice, though the sounds of you moving through such foliage can be heard if people are paying attention.
Strike during 'moments of action'
When you're trying to be stealthy, it doesn't do you much good to be the only one making noise. Taking a shot during a long stretch of silence tends to focus all of the enemy's attention towards you, reducing your lifespan drastically.
Instead, try to operate in a manner that allows you to maintain awareness of the enemy without forcing your hand. When the enemy becomes engaged or when loud environmental events happen, that's the time to make your move. If the enemy takes fire from elsewhere, a single rifle shot from you may easily be lost in the confusion. Don't get greedy - if you miss a shot, you may have to wait for another opportunity. It's a lot like sniping - a single shot is hard to identify, while multiple shots become more and more easy to trace the source of.
Looking is not necessarily seeing
One of the hardest skills to learn, and the most dangerous to employ, is the ability to distinguish the difference between an enemy who is scanning an area, versus one who has actually seen something during their scanning. In dark and dense terrain, when you're utilizing proper cover and concealment, sticking to shadows, and minimizing your movement, it can be tremendously difficult for the enemy to see you - even at very close ranges. It requires a great deal of self-control to be able to sit unmoving while an enemy looks in your direction or scans past you, but it can be the difference between maintaining your stealth and suddenly being thrust into a firefight you may not have wanted, on terms you didn't fully choose yourself.
When the enemy is moving, the situation is even harder for them. If you can blend in with the scenery and position yourself in a fashion that they're less likely to be looking straight at - for instance, on the opposite side of a tree that they're about to run past - you may be surprised at how many enemies can move by you without noticing your presence.
Being able to tell when you've actually been spotted depends on the distance at which you're facing the enemy. The most fundamental give-away is the double-take - if an enemy is scanning an area, sweeping their rifle over it, and suddenly stops and moves back in your direction - you may have been made! It's often very obvious when someone who wasn't necessarily expecting to see anything, suddenly sees something surprising - they'll jerk their aim towards it instinctively. At closer ranges you may even hear them exclaim something to their teammates. In the event that you're reasonably certain you've been spotted - kill them as fast as you can. A single gunshot will give you away to a degree, but it's not nearly as bad as having a living enemy able to coordinate with their teammates to surround and destroy you.
Once you've taken the shot, the situation becomes far more dynamic. You'll almost certainly need to reposition, but also keep in mind that the situation - particularly the availability of concealment and cover, and the proximity of the enemy - may make it possible for you to stay hidden even after firing that single lethal shot.
The Psychological Element
Keep in mind the psychological aspects of what you're doing, and how it appears from the enemy's perspective. If a fireteam moves past you without seeing you, any trailing elements will be naturally inclined to believe that the area is clear - else the fireteam would have taken contact, right? Use these assumptions to your advantage!
If you do get in a fight, try to get into the enemy's mind - what will they expect you to do? What do they think you are - a single soldier, or a full fireteam?
Patience
When all is said and done, most of your success at night while utilizing stealth will come from being patient, deliberate, calm, and thinking like the enemy. Rash decisions, panic, and hasty movements are your downfall. When you and your enemy are without night vision, just remember that the night is as much of a hindrance for them as it is for you - if you wouldn't be able to see something, neither would they. Use this to your advantage as you move about.
Weapon Usage at Night
Weapon employment at night brings with it hazards beyond what you'd experience in daylight. The muzzle flash of your weapon stands out brilliantly in the dark, allowing enemies to rapidly identify your firing position and return fire. Suppressors are essential to avoid this sort of signature, yet are often unavailable. In the absence of suppressors, be very careful about firing from solid cover whenever you can. If there's no solid cover, change positions after each burst - the enemy will return fire where they last saw the muzzle flashes, and you won't want to be there when the rounds arrive.
In addition to muzzle flash, tracers can be major liabilities at night. Weapons with significant launch signatures - such as anti-tank or anti-aircraft rockets or missiles - will give away your position the instant they're fired. The flash of an AT or AA missile launching can be seen from a great distance at night, and it tends to attract the attention of vehicles that could potentially be harmed by such weaponry. If you fire an AT or AA weapon at night, have a plan for where you can run or displace in the event that something bigger than you starts shooting back. If there's a risk of it, you're better off immediately displacing once your shot has been fired.