Both nights before going to bed I had seen the two fish that are now gone with no trace the following mornings. Still hear a click sounds in late day. My clown fish use to sleep on the sand next to the glass panel. Now the female is always inside the Elegance coral. So now it looks like one fish inside the DTCould it be a small pistol? I've had 2 mantis over the years, both clubbers. Neither ever attacked a fish. Inverts were in the 'light snack' category for both. Both would, however, burrow down into the sand under the rocks and smash at the glass on the bottom of the tank to try to get deeper. If you don't have paint on the bottom of the tank and can look up from underneath, a visible area of missing sand might indicate one is digging and punching. Just a thought to help try to figure it out.
You can also put a piece of raw meat on a skewer and drop it so the meat is on the sand near the front of the tank then move away so it cant see you. If its a mantis it'll probably be out in a few minutes after it scouts the area.
If you do have one and catch it, and would like rid of it, I'd be happy to set it up in its own place.
And, HMD
I can't look from the bottom since the tank is sitting on a pvc board.
"You can also put a piece of raw meat on a skewer" Meat as in beef or a clam? Do this when the lights are out for the night or can this be done during the day?