Stealth

Stealth is a mechanic which is first introduced in Update 7. It offers a better and easier time doing full stealth runs. This is only the first update of the like, so expect tweaks in the near future.

How to prompt an Assassination
As long as the crew is not aware of your presence, you are eligible for performing stealth attacks. In order to do so, approach an enemy from behind without alerting anyone to your presence, and press your melee key when the prompt appears. Depending on what melee weapon you are using, different animations will occur. Successfully pulling off a stealth attack will deal damage with a large multiplier, which is affected by the weapon's Charge Damage. So far, the animation varies by weapons type, where Daggers, Swords, Dual Swords, Heavies, Staffs, Polearms, Boomerangs, Gauntlets and Dual Daggers all have their unique animations.

Enemy Eligibility for Stealth Attack
Mechanical enemies do not grant a stealth attack animation as of Update 9.1, namely the list of enemies below. All other enemies can be stealth attacked.

Grineer Corpus Infested
 * Roller
 * MOAs
 * Ospreys
 * Cameras
 * Turrets
 * None (All Infested enemies can be stealth attacked)

Humanoid Bosses such as Captain Vor and Sgt. Nef Anyo can be stealth attacked, however the damage done will not be sufficient enough to kill them in one hit. Non-humanoid Bosses such as Phorid and Jackal will not grant any stealth attack animations.

Detection

 * Noise caused by breaking objects (containers, fans, windows), your unsilenced weapons or manipulating based abilities (Decoy, Pull etc.) will alert enemies. Opening doors, all movements of the player will not attract attention. Certain weapons make less noise (Paris, Kunai, Despair; or certain weapons with very good range like Latron, Snipetron and Vulkar, at the right distance), are less likely to attract attention.
 * Weapons with noise create "3D" noise, which was supposedly improved with Update 8.0, which was noted in the livestream. The bigger the noise, the further it travels, and the more likely it will alert an enemy. Banshee eliminates this noise with her Silence.
 * If not in cloak, bumping into enemies will cause an alert.
 * Their range of view before in alert is <180 degrees, so crouching still in a corner is a legitimate way to expose the back of a Grineer that just walked through a door. They are also bad at detecting objects at a distance and anything somewhat above them. Hiding behind terrain will conceal your presence.
 * When enemies are in alert, they will become highly sensitive to your presence. Their viewing cone will be wider, and will be able to spot you or your sentinel even from a very far distance, unless you are cloaked.
 * Performing an unsuccessful stealth kill will alert the victim (happens a lot when in a high level area, or on heavy Grineers with shields).
 * In order to put the whole level in alert, an enemy needs to activate a control panel. Killing everyone before a control panel is touched can still allow you to remain undetected. However, gunfire will put enemies in other map tiles in alert temporarily. These enemies keep watch behind cover, or patrol with their guns raised and are more sensitive to your presence. They will "cool down" and can be stealth killed after a short period of time.
 * Even when the control panel has been accessed and an alarm has been given, enemies will lose track of you when the last group of enemies you killed is not close enough to them to set them on alert. They will wander as if you were not detected and it is possible to finish a Stealth side mission this way.
 * Cameras make loud alarms when they see you, but again, if they are destroyed and nothing is in range to take advantage of their alert, you're safe. Rooms with two cameras are difficult to get past undetected without destroying them.

Silenced Weapons

 * Bows (Paris, Dread) are completely silent, able to multi-kill and pin corpses out of sight.
 * Lanka
 * Explosive weapons (Ogris, Bows with Thunderbolt) are oddly silent.
 * Acrid (need confirmation if ticks are silent when instant kill is not achieved).
 * All melee weapons, including the Glaive and Kestrel.
 * Throwing weapons: Despair, Kunai, Hikou.
 * Cloaking based warframe abilities (Smoke Screen, Invisibility).
 * Offensive based warframe abilities (Shuriken, Slash Dash etc.): silent only if cast under cloak.
 * Ash's Teleport: silent only if cast under cloak, otherwise teleporting to an enemy will cause you to bump into it and cause an alert

Tips

 * Be patient.
 * Crouch for better sneak runs. You'll have a smaller model so you can hide your silhouette better. The speed at which you move is also more convenient for timing your melee than running, and you will be less likely to bump into the enemy before the prompt appears. Also, it seems that enemies are less likely to notice you while you are crouching.
 * Stealth attack does not necessarily yield a kill. Although a prompt does appear, it is near impossible to stealth kill any heavy units, shielded units and enemies at high levels. Attempting to stealth kill these enemies is highly not recommended. Consider using high powered silenced weapons such as Paris and Kunai.
 * If enemies see a corpse that has not disappeared yet, they will be alerted for a short period of time.
 * If you have a Shade with Ghost ability, consider sprinting toward an enemy to activate it quickly.
 * Rolling is a great way to move quickly while crouching.
 * Beware of breakable reinforced glass, unless it is part of the player's tactic to cause a controlled lock down to eliminate all enemies inside a chamber. This action will not cause a high alert.
 * Having a Sentinel equipped may be detrimental to your stealth ability, as it does not hide behind low cover.
 * Although not considered silent, Ash's Blade Storm can potentially take out a whole room of enemies (number of enemies depends on mod rank) very quickly without causing any alert, thanks to its long range capability and high damage. (Probable bug: If you mash the stealth attack button while performing blade storm on an unalerted room, it is possible to get stealth attack damage in addition to the blade storm damage.)
 * If the prompt does not appear, consider doing a charge attack.
 * Kunai and Despair are good weapons to quickly eliminate a small pack of enemies without alerting anyone.

Current Grievances in the Mechanics
Since the concept of stealth is still a new one, there are some flaws that have not been addressed.
 * As of Update 9, the stealth attack prompt does not always appear for the Grineers due to their new idle animation. In the mean time, consider doing a charge attack.
 * The practicality of attempting assassination on heavy and shielded units.
 * If you aren't at the same altitude as the target, the prompt will not appear easily.
 * Prompt will appear and disappear unexpectedly when the enemy are in alert.
 * Not all enemies can be stealth killed. It is hard to know whether an enemy type can be stealth killed until the player approaches behind the target.
 * After each stealth attack, you finish standing up, which might reveal your position. You will need to release and re-press the crouch button to return to crouch position. If you are using a toggle crouch key, simply press it during the animation to return to crouching when the attack completes.
 * There appears to be no inherent benefits of stealth gameplay aside from the prompt for an assassination and lower enemy spawn rate, which is counterproductive given that weapons level via kill exp.
 * A glitch can occur when stealth attacking Infested Ancients. Upon entering the stealth attack animation against an Ancient, the Ancient will shrink in size, to around the size of the player.