I used to have 5 in my 30g tank. I think there's one left after 2.5yrs. But who the heck really knows? I could have 5 of them in there - just never seen 'em. They are always buried in the sand, so you just hope they are okay.
Usually, when a nessarius snail is really hungry, it'll come out of the sand to search for food. I've seen mine on the glass before and knew I was running the tank too "lean" (not feeding enough). I quick melted some Marine Cuisine in warm RO water and sucked it up in a turkey baster. Shot a couple pieces of meat in front of the snail. He snatched it, crawled back down the glass and burrowed into the sand bed to enjoy his meal.
Basically, if ya see em out during the day - they be hungry. They are a sand burrowing snail and would prefer to STAY submerged in the sand. They should come out of the sand to graze when you feed the tank, and then return to the sand. If they are out of the sand roaming around during the daytime, they need food soon.
But that's just my experience with my own ness. snails. I'm no expert.