Here's what Dr Ron has to say about Bumblebee Snails:
Engina species, including this one, are carnivores that subsist on eating carrion if necessary.
They are buccinid snails, and as such have a long proboscis that they can insert into clams, snails and worm tubes to rasp out the flesh of the prey. They likely secrete a poisonous "saliva" to immobilize their prey. Unlike the muricid snails, they do not have an "accessory boring organ." Consequently, they can not bore through calcareous substrata.
See this article for a discussion of predatory snails in general, but the article also shows the extension of the proboscis and feeding structures of a near relative of Engina that is found in temperate areas
They are preferentially predatory, that is to say they will eat living material in preference to scavenging food. If they do scavenge, they will preferentially scavenge carrion. Research on Engina species indicates, in nature, their diet preference is 1) worms, 2) snails, and 3) small clams. In a tank, there predatory activities may be obscured if 1) there are a lot of prey (such as worms) that are living in sediments (the hobbyist would simply not notice that they are being eaten, until the DSB started to fail) or a lot of snails. Additionally, as in all predatory snails of this nature, there are individual variations in dietary preferences, some individuals of the predator may prefer worms to snails, and vice versa. Interestingly, it turns out that they have to learn how to hunt, and will generally learn to eat the most abundant of their potential prey. Another point, though, they will always be predators.
Predators are not "found anywhere." Most predatory snails have both habitat and dietary preferences. The food for these more-or-less generalist predators would likely be found everywhere, and I suspect and differences in distribution would be due to other factors such as a prevalence of particular habitats or perhaps a differential distribution of their own predators. Also distributions vary through time. I have watched populations of some similar predatory snails, appear in an area, persist for about about 10 years, then vanish (in this case their predators became abundant), then after another 5 or 6 years reappear, presumably to start the cycles over.
As far as predators upon them in a reef system, I can't really suggest any that would be benign to other tank inhabitants. In nature, small buccinids are often eaten by small crabs, octopuses, sea stars, or some fishes.
