The only times I "see" the mantis shrimp in my tank is after feeding. If I wait 2-3 days between feedings - the chances are better it will come out of hiding.
If I wait a few days - then as soon as I feed the corals/inverts/fish - the little bugger shows it's face.
Weeks if not months will go by without hearing a sound - then it starts up again - popping away.
I've lost several emerald crabs, 2 peppermint shrimp and a cleaner shrimp in the past two years that it's been in my tank. Although I did not get a good look at it until much later.
I've finally ID'd the rock(s) it lives in, I think, and plan to remove the rock and little bugger once and for all.
Waiting for my new live rock to get here, then cure it, then when it's ready to go into the tank - it's mantis removal time.
Until then - no more crabs or shrimp going into this tank.
reefin_with_jesus,
You might want to try looking for it after feeding frozen foods, push a little crumb near it's hiding place with a long plastic rod or something similar. It just may show it's nasty self to you then.