This can be caused via grenades or other collateral damage.
If a member of the XCOM Squad stands within a tile that would rescue the civilian the Faceless is disguised as.
Faceless will reveal themselves under the following conditions:.
Leap: The Faceless can jump to higher surfaces such as roofs and cliffs.
Regeneration: The Faceless can automatically self-heal at the end of their turn.
Claws: The Faceless' only attack is an area-of-effect (3 tiles) melee swipe with its claws that can damage multiple units and destroy cover.
The threat of this unit being hidden among the general populace in conjunction with civilians having a chance of removing concealment from the squad makes not only civilian rescue missions, but civilians in general much more of a liability. The Faceless can move both farther and with more agility than may be expected from their size and lumbering gait. Unnaturally tall and consisting of a gelatinous fluid, these alien foes have significant reach with their claws. This unit disguises itself as a normal-looking civilian until provoked, whereupon it transforms into its true, alien form.