There are different kind of secret triggers in minecraft: Entity movement -------------- Living entities like animals and monsters can only move if a player is in a 32 block radius, this radius is a spheroidal box around the cow. By placing the a cow in a place where pistons keep pushing the cow back you can make a proximity detector. This can be seen at: https://www.youtube.com/watch?v=3hbnXDt7xsE Tripwire -------- If properly applied, tripwires are hard to see, this works the best if the tripwire is placed around the eye level in buildings, or at the feet in the outside world. Hidden pressure place --------------------- Its also possible to make a pressure plate completely hidden, for example, stone pressure plates can be nicely hidden in a hallway that goes down, the following image demonstrates this: [![enter image description here][1]][1] Did you see the pressure plate at the end of the stair? Block update detector based detection ------------------------------------- Some blocks have a special property that they update when the player walks over them, for example, redstone ore blocks start glowing. This can be detected with a BUD to deliver a redstone signal. A simple implementation this looks like: [![enter image description here][2]][2] The item through a corner technique ----------------------------------- Items can be pickup through the corners of the blocks, by making your player pass past such corner, you can detect it using a wooden pressure plate in combination with a torch. The casual player technique --------------------------- What if you didn't need to make a hidden trap? You could add a couple of iron doors in combination with buttons to your building, this way the player going through it will thrust the button to continue, and keeps pressing them to open the next door. The trapped chest ----------------- By placing a large number of loot crates through your building, player will get blind opening chests, they will just open every chest to get a little loot. By making the trap have a trapped chest in combination with some delay, the player will suspect nothing. The minecart in a corner ------------------------ Like items, you can also interact with minecarts in corners, by combining this with a powered rail and a detector rail, you have a accurate corner touch detector. [1]: https://i.sstatic.net/Tdx7L.png [2]: https://i.sstatic.net/ecOz8.png