They will wedge themselves into a notch, recess or hole in rock, making them so easy to miss with the rock out of the water. Get a clean plastic garbage pail that you can carry when half full of tank water. Then you need to be meticulously thorough by searching (doing it outside makes it easier). The sunlight helps in 2 ways - 1) You can see better (wear high contrast yellow tinted sunglasses if you have any). 2) As you rotate the rocks the shrimp (and crabs) will try to move out of the bright light. That movemen makes detection much easier. Another pair of eyes always helps. I highly recommend that you wear gloves. Not so much for the shrimp but for other critters that can sting plus to reduce the risk of cuts. I learned the hard way with bleeding like mad without realising that I've even been cut. There are stories of nasty infections too.
With traps (read below), I've had fairly good success catching pistols and mantis from my and friends tanks. Mantis seem more likely to enter the trap but they are smart enough to escape. One time a mantis actually busted open the side of my plastic trap by punching a hole in it.
For pistols/mantis, a plastic soda bottle with its top cut off then placed inwards gave only fair results. Better results came with using a plastic food container weighed down with some sand or small rocks plus the bait inside. Cut an "X" (about 4" x 4") on the lid with a sharp Stanley knife then bend the 4 triangular tabs downward just enough for the shrimp plus its pistol/club to get in. Getting out will be much harder. Pistol shrimp have poor vision. They usually rely on buddies (watchman Gobie) for the all clear before venturing out. If you have a watchman, I recommended that you leave it in the tank. It will give the pistol shrimp a better sense of security during lights on. Setting the trap an hour before lights out seems to be the best way. It's your choice of bait, but an average sized shrimp or some mysis in mesh are both tempting. Mysis seem to give off a stronger scent and can attract other life not on your hit list. You can always get them out along with your problem crustacean.
I use a small blue LED flashlight to scan the trap about an hour after lights out. I let my eyes get used to the darkened room for a few minutes before scanning. It really helps with our night vision.
Good luck and post photos of your pesky mini Clint Eastwood.
