Look carefully for the proboscis. Usually when I feed the tank, they all emerge from the substrate when they sense the food. They are like snail commandos.
Nassarius tends to sit still in sand until you put some food in the water. They are carrion eaters. They will turn up in a hurry when frozen food hit the water just like Markkasdad said.
I have no idea. I think so, because I can't find them. I'm going to do another water change tomorrow and go ahead and place an order with reefcleaners.
Okay here is going to be a million dollar question for you jwilson. How do you check your salinity? Do you use a Hydrometer or Refractometer? And where is your levels according to the device you use? I actually just wrote an article on this subject and submitted to David here at R2R for publication on this matter.