Eye Sigil ARG/Mini Metro
Mini metro is a game developed by Dinosaur Polo Club, available on Steam, GOG and iOS store. It was fully released 06 Nov 2015, but was available in early access long before that.
The sigil was added in an update on 13 Feb 2015 (the game was still in Early Access).
Sigil appears in the background after going to the credits screen, going back to the main menu and going to the credits screen again.
Clicking the sigil opens the secret map called Canberra.
Sigil appears both in Day Mode and Night Mode, but it's more visible in the Night Mode.
The player has to solve the Canberra level by recreating the sigil. This is a bit tricky, though, since the solution requires few of the tracks to have T-like connection, which is not possible by the normal rules of the game. This is done by using the "ghost lines" — tracks that have trains still running on them when the track was rerouted.
The piece appears on a popup on screen as soon as the correct arrangement is achieved.
Discovery process notes
The Canberra map was discovered in May 2015 by the community of Mini Metro players.
On 24 Jan 2016, when the GameDetectives were dissecting the game, user /u/nint8835 has modded the game to return true from the function checking if level is solved correctly and got the piece. The legitimate solution was discovered later.