You mean the green dots that appear when you come near the warps? Pretty simple.

D +-
D +-

D tiles are trigger scenery, the animations being two frames: frame one should be blank, and frame two should be a green dot or whatever. The + tiles are On trigger zones with the same TriggerID (higher than 0) as the D, and the - tiles are Off trigger zones. When you approach the dots, they will appear, and then disappear if you move away.