Sentry ward

Sentry wards (also known as sentry totems) are totems that can be used for spying, or detecting enemies in stealth. When placed, some sentry wards become invisible to the eyes of the enemy; unless they have a way of detecting them.

They are most commonly used by trolls and can be used to attune creatures to one's sight. Through this mysterious ward, witch doctors can keep watch over important areas for as long as the ward exists. Even though the wards will vanish in time, they are invaluable tools that enable the witch doctors to keep a constant eye on their surroundings.

In Warcraft III
In Warcraft III, sentry wards are mainly employed by troll witch doctors, although can be bought by Hero units.

When Thrall and the Horde landed on the Darkspear islands, he sensed a strange magic around them and using his far sight, he discovered several sentry wards.

In World of Warcraft
Sentry wards have seen a decent amount of usage in World of Warcraft though many occasions have been removed from the game.

Classic

 * From Classic and up until patch 4.0.3 of Cataclysm, shaman players were able to use the Sentry Totem ability, which was commonly used to either spy on enemies in Battlegrounds (knowing enemy movements ahead of time) or used to take screenshots, similar to Far Sight.

Zalazane's Fall

 * During the Zalazane's Fall world event, Vanira used sentry totems to create frog spies by having an adventurer attune the frogs to her sight using the totems.

Mists of Pandaria

 * In the quest Sentry Wards (quest), Shademaster Kiryn states that a "troll... friend" of hers gave her a set of sentry wards to use to spy upon the Alliance at Lion's Landing in Krasarang Wilds.
 * On the Isle of Thunder, Zandalari shaman had put down sentry totems across Stormsea Landing. During the quests A Bold Idea & Decisive_Action, an adventurer aids Taoshi with disabling them by using smoke bombs thrown from the back of a Thunderwing skyscreamer.

Warlords of Draenor

 * Sentry wards deployed by a goblin by the name of Ketya Shrediron were used to guard her hideaway located in Talador.

In the RPG
The gnoll Rerjik Mashnose has a sentry ward.