There Came an Echo
There Came an Echo is a real-time strategy game with a heavy lean on voice controls developed by Iridium Studios and released on 24 Feb 2015.
The sigil was added to the game on release.
The sigil is visible on the wall to the far right side of the level 5, near a point called Foxtrot 6.
Datamining the sigil showed that its texture also has a formulae. (TODO: people also saw it in-game at the Foxtrot 6 point, but how?)
Saying the voice command Old McDonald Had a Farm while standing at the Foxtrot 6 points flicks the piece on the screen.
Discovery process notes
The game has a bunch of easter eggs triggered by saying particular voice commands. In May 2015, three monts after the game release, the developer posted the list of the secret trigger lines including the screenshot of the sigil and a vague message about noone finding it.