Cryptic :
[01] A fire burns above you.
[02] A candle o'er the skyline visible from the first steps.
[03] A King and his son sit on a high path between the second and fourth.
[04] A second sacrifice was made.
[05] The Eye of Corruption sits within ruin.
[06] A dark corner hides a brooder. They frighten him.
[07] High. Higher still. Still higher.
[08] On the roof of a warehouse that stores no wares.
[09] A hex pulses in the shadow of industry. Will you be bewitched by the beat?
[10] A hidden path through a daredevils trial. Descend where others soar, soar where others fall.
[11] Do machines ever get thirsty?
[12] Dive, but don't turn your back on this killer.
[13] A golden structure hides a destitute secret.
[14] Below a golden machine, a very different beat plays.
[15] You see the goal, but are you ready?

Detailed directions for those that don't want to go hunting (or spend 20 minutes watching my deliberately slow replay):
Secrets 1-9 are in the city, above ground.
[01] Near the start, look for a burning building on the left. You should see this one if you gripfly towards it from the first jump
[02] Look straight ahead down the first straight. There's a pillar of light coming off the top of a building, past the first crossroads. Climb up there.
[03] Turn left at the first crossroads. Instead of completing the transfers, climb the building with the virus node on it. You may find this secret accidentally trying to plot a route over the buildings between the two virus alters.
[04] On the slow route, there's a fallen building that you have to drive on the side of to progress. Just after that building there's a second 'Virus alter' on the right. Blink and you'll miss it. Easier to spot from the air.
[05] When driving on the fallen building, look left. There's a ruined building in the background aflame. Fly to it. Using the teleporter is optional.
[06] Still on the slow route, stop in front of the tunnel entrance. Climb up the left and there's someone to meet hiding on a small ledge.
[07] If you take the fast path, there's a floating junction visible against the moon - it's only there to signpost the high path behind it. However, the tower with the poster *does* hide a secret. Climb to the top - you'll need to get right on top of the tower, which can be tricky as parts of it move. This secret also comes with a bonus if you get close enough to it - and a punishment.
[08] Probably the hardest one to find without help. On the slow path before the waste plant, there's a huge building to the right with cables entering it. You don't want to enter it, but instead climb on to the roof. There's someone to meet, but you probably won't spot them until you're right on top of them.
[09] This one's easy. Turn left in front of the waste plant.
Secrets 10 and 11 are in the underground area after the tunnel with all the transfers.
[10] In the waste plant, there's a hidden exit. Instead of completing the jump, let yourself fall half way. A purple light is a marker. This path ends in a 2d transfer down, but if you head up instead there's someone hiding behind the building. There's an optional teleporter here as well - just step on!
[11] Stop at the 'INFECT' sign. Look around on top of the generators around you.
Secrets 12-14 are on the secret 'fast path' after the gripfly section. At the floating junction and tower, there's a secret, fast griply route to the left of the tower part way up. Follow this route until it dives into a tunnel
[12] Turn around before you exit the tunnel into the yellow room. You'll need to pop off the road on to the tunnel walls, but there's a fishy frenemy to meet.
[13] In the large dark room with the yellow track, stop when you see the first pipe over the track. If you look back, there's a vertical structure with hazy lights; there's someone to meet at the base of this structure.
[14] In the same room, descend until you come to a section with multiple pipes and arches across the track. Drop from the track to the floor, turn around and follow the edge of the room around. This one's pretty bright so should be easy to see when you're the ballpark.
Almost there
[15] A door at the end of the level opens to reveal a tunnel with an endzone; enter the tunnel, but drop under the track instead of continuing.