All secrets are indiscreetly marked with a Lambda symbol in the background.

Here are hints for finding them if you’ve missed any. They are also shown in the walkthrough video below.
Secret Location #1 (1:06)
Hint#1Level 2, after checkpoint 3
Hint#2This is a leap of faith
DetailsThe Lambda is a warp, drop down and touch it from the platform (where the red ball drops)
Secret Location #2 (3:03)
Hint#1Level 4, after checkpoint 1
Hint#2This is a leap of faith
DetailsThe Lambda is a warp, drop down and touch it from the platform (where the stairs descend)
Secret Location #3 (8:10)
Hint#1Level 7, after checkpoint 10
Hint#2This is the barracks
DetailsWhere the text “Barracks” in a room below a platform, a portion of the wall is missing that can be jumped through
Secret Location #4 (9:32)
Hint#1Level 9, after checkpoint 2
Hint#2Not all platforms are visible
DetailsFrom the second step on the stairs, jump up in the opposite direction to find an invisible path
Secret Location #5 (10:01)
Hint#1Level 10, after checkpoint 1
Hint#2Go right instead of up
DetailsWait for the laser and then head to the right and jump up to the Lambda warp
Secret Location #6 (13:28)
Hint#1Level 13, after checkpoint 7
Hint#2This is a leap of faith
DetailsThe Lambda is a warp, drop down and touch it from the platform (between the two guns mounted on the ceiling)