Try staking out the tank one day and one night. At night don't use any lights, just have enough light that you can see some detail in the tnak. red lights are useless for mantids. Beer helps
. Just sit and watch. Search the net for some mantis pics if you don't know what they look like.
The only hard evidence of a mantis imo is a visual, as not all mantids click and not all leave shells in the open. Not all hunt fish either. but anyways describe the holes in the shells. Are they perfect circles/ovals? or jagged like they were smashed in? Mantids would leave the jagged kind of hole.
Did you just add LR or something that the mantis might have just hitched in on? It seems very odd if it is a mantis because your shrimp and hermits survived just fine for this long.