I was able to trap mine with the coke bottle trick.
take a 2 liter coke bottle, and cut the top off about 4 inches below the opening. insert that part into the bottle so you basically have a funnel inside a coke bottle. silicone it in place, and leave it for a day. then cut a trap door in the wall of the bottle. by this i mean cut an opening in the wall, but only cut 3 sides... leave the 4th side attached. this will allow you to get the mantis out relatively easily. do not bend open the trap door, leave it as is, until you are ready to take the mantis out.
after is is all dry and you are ready to go, use a piece of raw shrimp to bait the bottle. place the shrimp in the bottle and submerge the bottle in your tank. leave the bottle with the neck end facing the area where you heard the mantis snapping. i had to weigh my bottle down with a piece of live rock because is kept trying to float.
anyway, i caught mine in a few attempts. dont leave the bottle in there for more than a day or two or you will start to polute your tank with that shrimp decomposing.
basically, the idea of the bottle trap is to give the mantis a surface to walk up and get into the bottle, but it wont have anything to walk up to get out of the bottle. hopoe that makes sense...
~mike