Alarm

In the Deus Ex series, an alarm is triggered when the player fails to maintain stealth in a hostile area. Alarms generally draw in additional hostile forces and may also cause civilians to panic. Depending on the particular game, an alarm may be triggered when a hostile character spots the player and activates an alarm panel, or when the player is spotted by an automated security device, such as a security camera, laser tripwire, robot, or turret.

Deus Ex
In Deus Ex, alarms can occur when an alarm panel is activated (generally by a hostile non-player character or a laser tripwire), or when the player is spotted by a security camera, which has a built-in alarm. Alarm panels and security cameras can be destroyed by high-damage attacks, or disabled by using multitools. They can also be temporarily disabled via an EMP grenade. Security cameras can sometimes be disabled by hacking a computer controlling the camera.

Some alarms are not tied to any specific security device. For example, after JC Denton hacks the main terminal at the Hong Kong MJ12 Labs, an alarm will automatically sound even if he was not spotted by any guard or device.

Deus Ex: Human Revolution
In Deus Ex: Human Revolution, Adam Jensen can hack alarm panels to disable them, so that they can no longer be used to sound the alarm. Guards will still attempt to use disabled panels, but nothing will happen when they do. Some objectives, if completed without triggering an alarm, with award a "smooth operator" bonus. The achievement Foxiest of the Hounds is awarded if the game is completed without causing a single alarm.

Deus Ex: Mankind Divided
In Deus Ex: Mankind Divided, the Foxiest of the Hounds achievement from Human Revolution returns, and is likewise awarded if the game is completed without causing a single alarm.