The Stranger
Though the Outer Wilds' DLC Echoes of the Eye opens with a bit of a puzzle the DLC doesn't truly begin until you land aboard the mysterious vessel known as "The Stranger". Spoilers for the location follow below.
Once you've safely landed within one of The Stranger's two hangar bays you can unbuckle your seatbelt, put on your suit if you haven't done so already, and exit your ship.
Upon exiting you should be faced with two adjacent doors. Feel free to enter either one as they lead you to the same place. Inside you'll find a room with white markings on the wall and a totem pole in the center.
Pull out your flashlight and shine it on the fins of the totem pole as this will cause the fins and the white markings on the wall to rotate.
Shine your flashlight until the markings on the wall all line-up and when they do quickly turn off your flashlight to solve the puzzle and reveal either a secret room or a secret riverside entrance.
If you're faced with a dark room, you'll find four green orbs and an interactable interface that says, "Rotate" when you approach it. Rotate the wheel and you'll plummet into a flowing river atop a raft.
The four green orbs surrounding you are now your raft's steering mechanism. Aim your flashlight at the orb facing the direction you want your raft to move.
After a few seconds of floating down the river, you should come across a shining light that somewhat resembles a tractor beam. Steer your raft underneath the beam and a mechanism will lift you and your raft to the dock above.
Alternatively, if you entered The Stranger through its second hangar bay you will solve the totem puzzle and see a raft at the edge of a river. Push the raft and jump aboard to follow the current.
You have now entered the area named the "River Lowlands" and after taking a few steps towards the mainland your Ship Log should update once again.
If you enter through The Stranger's hangar that brings you to the location pictured above the hangar hides an additional entrance. Instead of walking into one of the two doors work your way to the left.
You'll find a darkened passage that has an elevator that will lead you to a different area. If you explore this area you'll find some schematics near a window that faces the sun and a staircase that leads directly to the top of the "Reservoir".
You will become better acquainted with other areas of The Stranger in the following pages but there are no other secrets to be found within The Stranger's entrance area.