Guard Dogs will often accompany normal Guards and other enemies. They are generally harder to sneak up on than normal enemies due to their keen sense of smell which is of a large radius, and is not fooled by darkness, hiding places – or even the path traced by the Mark of Serenity. The actual arc is a 180 degrees from front, to above, and behind – but not beneath them, making them vulnerable to stealth takedowns from floor grates.
Guard dogs that smell the player will turn to investigate the source of the smell (just like distracted guards). Guard dogs that see the ninja will start barking to alert other guards to your presence. By itself this will not raise alarms, but can result in them if human guards see the player after the dog starts barking. Human guards who discover the body of a defeated Guard Dog will also not raise the alarm in response.
Although a barking dog will not raise alarms, it causes the dog to become alert, effecitvely causing you to lose your No Alarms Raised Bonus at the end of the mission.
While they do not have guns, dogs will try to bite the player, which can deal damage.
Like their human counterparts, they can sometimes be encountered in an intially asleep state. This drastically reduces the minimum safe distance at which they can be approached, as they don't sniff out their surroundings while asleep. Stepping on or past them, however, will awaken them – as will all other in-range Distraction events.
Guard Dogs that are dispatched will grant the bonus for Hound's Slumber.