Eye Sigil ARG/Flickers
Flickers is a free timing-based 2D platformer game developed by Neat Snake. It started as a student project for DigiPen course GAM 250 and was released on 01 May 2013. It is available for download on the developers website.
The sigil was added in an update on 11 Sep 2015. (Note: there is no sigil in the old version of the game on the DigiPen website)
Sigil appears on a tree in the background shortly into the game. There is no level indication in the game, but judging from the game files, it's forest level number 4.
There is a circle on the sigil level that disappears after a very short time (about 3 seconds). Piece appears when reaching it in time. You cannot take the "normal" designed path of the level as it is too slow, and need to jump left from the checkpoint and then immediately makea a long jump diagonally up+right to the much further node.
Discovery process notes
The piece was hacked initially by modifying the level data — moving the triggering point and hitting it in the game. This was prefaced by reversing the level format, writing tools to convert it back and forth into XML and noticing that a certain thing was only present in the sigil level. The legitimate solution — hitting the trigger where it is placed — was discovered shortly thereafter.
The developer joined Discord himself and chatted with GameDetectives for a while after the piece was retrieved.
The sigil is one of the textures on Forest6 sprite sheet. It is referred in Forest6.json as
sigil.png It is used on the level described in forest_paul4.blvl.