Well one thing to consider: if it is a mantis you would see crushed shells (fragements anyway) and not too many dead bodies. When "smashing" stomatopods go after a hermit/snail they generally will smack the shell (which would leave a little bit of evidence behind), kill the inhabitant, then drag the body into their home, eat, then bury what they do not finish thus saving the rest for future use/tank parameter problems. Which would of coarse leave a trail behind (i.e. shell frag's). Just my experience of coarse,
Brian