How many turbos do you have and how old is the system? Snails have a very high mortality rate if there is not a plentiful food source, and they will die in droves. If you find the shells empty, that doesn't mean they aren't dying from other causes... it just means your janitor crew is doing it's job.
If the shells are not damaged, then it's not a mantis. If something is killing them, it's also spending time eating them, and a snail is a big hunk of meat, which takes a while to eat. Check the tank at night repeatedly with a small flashlight to see if you can identify a culprit. It could be that you have a rock crab or some other hitchhiking predator, but they may just be dying on their own.
Don't be surprised if you see a hermit crab or cleaner shrimp going at it though... but that doesn't mean they are the killers. They may have just died of natural causes. However, if something is killing them, it's a cinch whatever it is is going to stick around to eat it.