Monotremes have skin covered beaks that have small mucous glands (from 40,000 in platypus down to 400 in short beaked echidna) around the tip and outer edges, and the snot conducts any weak electrical signals to a receptor at the back. Now, this is easy enough for the platypus, since it forages underwater. But what about echidnas?
Apparently, they like digging through moist earth. If the earth is not moist enough, they dribble snot everywhere from their constantly runny noses.
As well, their snouts have mechanoreceptors to help feel tiny motions in the mud/dirt. They can also engorge the area with blood to make the cells stiffer and even more sensitive. Yeah, they actually get nose boners while foraging.