Smooth Operator

Smooth Operator is a stealth-related XP bonus in Deus Ex: Human Revolution. It is earned by completing a level without setting off an alarm and worth 250 XP. Alarms are generally caused 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. Finishing Deus Ex: Human Revolution without setting off any alarms will earn the player the Foxiest of the Hounds achievement.

Events that cancel the Smooth Operator bonus

 * Alarm panel
 * Activated by a human enemy or a civilian
 * Camera
 * Spotting the player (showing "Alarmed" or "Hostile" state)
 * Spotting a body (showing "Alarmed" or "Hostile" state)
 * 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
 * Computer/Security hub/Dedicated terminal/Alarm panel
 * Player failing a hack

Events that do NOT affect the Smooth Operator bonus

 * Scripted alarms (Sarif Plant, Tai Yong Medical Penthouse, Singapore)
 * Bots and turrets spotting the player, whether they show as "suspicious" or "hostile" on a 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 B-line 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.
 * 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). 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.
 * 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 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 shutdown 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 shutdown 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.