Alarm

Alarms are a technology featured in all Deus Ex games. They can affect entire buildings and complexes in many ways, but they all involve a switch or button placed on a wall in a high-security location, with guards nearby. When pressed, alarms will sound a loud noise throughout the area, alerting people and free roaming bots to their location. Alarms will also activate turrets and any bots hidden behind doors. Nearby armed guards will run for alarms when they see the player, or hear something suspicious such as a gunshot or explosion. Unarmed bystanders will run for alarms when they hear a strange noise, or witness the player attacking someone the bystander is sympathetic to. Depending on the circumstances, guards may run for alarms if they see the player breaking and entering a building or hacking an electronic device. Getting caught by a security camera for long enough or tripping a tripwire will also automatically sound alarms.

Deus Ex
In Deus Ex, alarm panels can be bypassed using multitools. If a guard attempts to use the bypassed panel, no alarm will sound. Alarm panels can also be permanently destroyed with powerful attacks, typically explosions, in the same way one would destroy a camera or a turret. Destroying or bypassing an alarm panel while the alarm is still sounding will stop the alarm.

However, after JC Denton hacks the main terminal at the Hong Kong MJ12 Labs, even if they he is not seen by anything, an alarm will automatically sound, and all personnel will know to attack JC.

Deus Ex: Human Revolution
In Deus Ex: Human Revolution, Adam Jensen can hack alarm panels to disable them (and obtain typical hacking rewards), 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.

There is also an achievement/trophy for not causing a single alarm.