By using redstone ore as the BUD (Block Update Detector) I have made a hidden fall trap. The BUD is silent so there is no warning at all. This trap self resets as well as auto collecting any loot from the victim, placing it in a chest. Works for walking and sprinting.
The only down side to this trap, is that redstone is usually found below level 16. You can always silk touch the ore to move it around, but players may be suspicious if it's found too high up. Alternatively a lava blade can be used to add some extra damage on the way down.