Take the first teleporter, and clean the area out a bit. There will be a flight of stairs; at the top, you have to jump over a gap. On the right, a secret door, but... it won't open until you have jumped out of the other end of this room...

Jump out, check that the door has opened behind you, then drop down, go to the stairs and do it again.

...Now you have found #1, look around inside. Doesen that wall have a funny greenish tint? I wonder why... Tap and enter.

Continuing around the path, there is a jump more, directly opposite #2. When you prepare to jump, watch your six. The wall, just to the right of your landing point, look greenish too...?

Just before you reach the exit, there is a small hole in the fence. Go through, and get the last secret.