Eye Sigil ARG/Reagan Gorbachev
Raegan Gorbachev is a top-down shooter/stealth game developed by Team2Bit and released on 24 Feb 2016.
The sigil was added on release as it was discovered by the players on day one.
The sigil is on the floor of a hidden room in Level 22.
Find the rocketlauncher and shoot the right wall at the end of that level to get to that room as seen here.
Finish the room and go through the top door to see the piece as shown here