Jump on the first pendulum, turn around and look beneath the platform you just jumped from.
Left side, up the ramp, near the end of the hive tombship jump puzzle secret chest.
Annihilator Totem left room 2nd floor.
Warpriest right balcony.
At the last hole of the maze, jump over and look back into the hole.
In the lower left hidey hole, beneath the platform.
Piston wall jumping puzzle, beside secret chest.
End of piston wall jumping puzzle, above final door.
In final room, above entrance