Smooth Operator

Smooth Operator is a stealth-related XP bonus in Deus Ex: Human Revolution. It is awarded along with "Getting Things Done" XP bonus for completing certain main mission objectives without setting off an alarm and worth 250 XP. Alarms are generally triggered by hostile electronic equipment: alarm panels, cameras, laser grids and computers of different kinds. Smooth Operator bonus is awarded as a recognition of player's skill at avoiding them. It does not require the player to be completely unseen by humans or bots.

Finishing Deus Ex: Human Revolution without setting off any alarms will earn the player the Foxiest of the Hounds achievement. For a complete list of all objectives that can reward a Smooth Operator bonus, see the Foxiest of the Hounds article.

Ghost is another stealth-related XP bonus in Deus Ex: Human Revolution (with its own set of rules).

Events that cancel Smooth Operator

 * Alarm panel
 * Activated by a human enemy or a civilian (or by you if done on purpose after hacking it)
 * Camera
 * Spotting the player (showing as Alarmed or Hostile on radar)
 * Spotting a body (showing as Alarmed or Hostile on radar)
 * Destroyed by a weapon/explosion. Note that EMP explosion doesn't count and can disable a camera safely and permanently.
 * Laser grid
 * Triggered by the player
 * Triggered by an object
 * Laser emitter getting damaged. Unlike cameras laser emitters will trigger an alarm if destroyed by EMP explosion. However, a single shot from stun gun will disable them temporarily for 10 seconds without any repercussions.
 * Computer/Security hub/Dedicated terminal/Alarm panel
 * Player failing a hack

Events that do NOT affect Smooth Operator

 * Any action that cancels Ghost bonus. Meaning that player can be seen and shot at — as long as human enemies don't get to an alarm panel Smooth Operator (and consequently the Foxiest of the Hounds) won't be affected.
 * Scripted alarms (Sarif Plant, Tai Yong Medical Penthouse, Singapore)
 * Bots and turrets spotting the player, whether they show as "suspicious" or "hostile" on radar
 * Cameras briefly spotting the player (showing as "suspicious" on radar)

Strategies

 * In general, either speed or stealth is needed; camera and alarm panels never trigger an alarm instantaneously and you will always have a chance to prevent it, be it from a camera becoming suspicious (yellow light) or an enemy making a beeline for an alarm panel. Be careful around laser grids or, better, use the invisibility cloak to pass through them. In case of computer hacking just don't make stupid mistakes: if you're not sure you will be successful, then disconnect and initiate a hack again — it is faster than reloading a save.
 * Also, keep in mind that failing a hack will count as an alarm. If you're locked out for 30 seconds, you failed the hack. The issue with this, is it means ANY hack, alarm panel in Picus, or personal computer in Sarif Industries. If you fail a hack, it will cost you Foxiest. Note that reloading will NOT cause you to fail this achievement.
 * If unhacked alarm panels are nearby, never use weapons that aren't silenced unless you have the means to quickly gun down everyone showing up on your radar, including civilians (who will possibly run off and alert distant enemies, or use the panels themselves). Non-lethal take-downs are the only 100% silent way to remove an enemy, and even silenced weapons generate enough noise to be heard at point-blank range.
 * Reduce Wall-punches to a minimum, or at least be extremely cautious of your surroundings when attempting to do so, as destroying a wall this way generates a lot of noises that can and will attract attention, potentially setting off an alarm (if already in an alarmed state). Be aware that the sight of debris will also cause an enemy to investigate and initiate a search.
 * Remember that the stun gun can temporarily disable cameras and other security devices without causing an alarm. This can be useful to give you extra time to move a body; or, if a camera sees you and you have no time to get into cover, and no energy for the cloak, for a last ditch attempt you can rush the camera and quickly "stun" it, giving you time to get away.
 * Don't forget that you can "throw" bodies great distances if you activated the Move/Throw Heavy Objects augmentation. It is possible to cloak and throw a body out of a camera's view, even upon becoming suspicious, before it sets off an alarm.
 * It is wise to upgrade your capture software to enable you to shut down turrets and bots; while getting their attention does not cause an alarm directly, the gunfire will get anyone's attention in the room. There is no reason not to hack and disable most alarm panels, however if you cannot do this safely, consider placing gas mines near them.
 * Obviously, having an upgraded cloak can make cameras and bots trivial, but do not forget to move bodies quickly. The cloak allows you to move through security lasers freely. In lieu of this, you may drag a guard near the lasers and they will shut down as long as the body is in proximity.
 * Enemies who spot bodies will often set off alarms; in most cases however they will investigate the body first, giving you a chance to neutralize them or move into a strategic location.
 * If you perform a take-down while cloaked, you will uncloak for the animation but since the game essentially pauses during this time, you will not be spotted by enemies or bots/cameras. As soon as the animation is over you are re-cloaked, and you will not be seen even if you are in direct line-of-sight. This can be useful for quickly dealing with an enemy that's about to trip an alarm.
 * Gunfire can be used to draw enemies away from their positions while they investigate. If using this tactic, remember to always fire while you are out of any enemy's line of sight, even if you're cloaked or else they'll see your gun shooting. Also note that they will inspect where the bullet ended up, not where it was shot from.

Bugs
There is a bug present in both console versions and the recent PC build (v1.4.651.0) of the original game, including The Missing Link (1.0.62.9), where (among other things) some Ghost and Smooth Operator bonuses may not be awarded to certain players when they complete an objective stealthily (see this video for an illustration). The details about causes and occurrences are not clear, but it seems that it has something to do with save game files of former playthroughs. If you're not getting a bonus you think you should get, you're most probably affected by the bug. It is strongly recommended to remove all save game information before starting a new playthrough. For PC users, the files are located at .../Steam/userdata/*UID*/28050/ for the original game and at .../Steam/userdata/*UID*/201280/ for The Missing Link. To make sure the data is permanently removed, disable the Steam Cloud for Deus Ex: Human Revolution. For affected Console users, creating a new profile for each playthrough seems to be the way to go.

However, this bug should not affect your completion of the Foxiest of the Hounds achievement. It will make it more difficult to track, however, and require a more meticulous attention to your actions.