I also just drop my shrimp in, no problems. I would suspect a mantis too, as they seem to be able to more easily get shrimp. Hermits are tough, I've never lost one to a mantis. I had a mantis banging on a snail the other day, no luck there for mantis either. Luckily I saw it and it was in a small isolated rock, which is now in quarantine in the backyard.
I've discovered new mantis and crabs in my tanks after years of running, so it is possible to have a long running tank and still miss stuff.
It could also be the 6 line, but you'd probably notice aggresion as soon as it saw the shrimp, not in the middle of the night.

My sixline never bothered the cleaner shrimp that was in the tank before him, but went after a few other inverts that I introduced after he was in the tank. Never killed anything though(he's still just wee).

Justinl I thought shrimp because that was the same way I found out I had one. I actually caught the third shrimp getting killed . That was when I realized it was a Mantis shrimp in my case. I also diidn't read that he had his tank running for a few years and yes he would have seen it if there was one. I would also lean towards the Acclimation if it is within hours that the shrimp are dieing but it sounds like he is taking every percaution there. I have a sixline that has taken out all my sexy shrimp.:twised:

Well I'm sure there is no mantis shrimp in this tank. The new shrimp was small and no sign of a corpse. In the past the shrimp I added were in the tank for up to a couple of weeks. I figured the got attacked while they were molting and vulnerable. They were also larger. I'm very carefull to acclimate any new critters. The sixline wrasse just kind of has a predatory look to it and he's quite large. I suspect that he is the murderer! I guess as long as he's in the tank, shrimp are out of the question. (p.s. I'm a she not a he) :)