Their presence is only noticeable through the disappearance of socks.
NB. This is never a pair of socks, only one (leading to the current odd sock problem across the globe).
The Snadwiches breed and raise their young in said stolen socks. When more and more socks go missing in a household, it normally suggests a 'Snadwich Infestation', for which a 'Snadwich Exterminator' must be contacted. Failure to do so could lead to a 'Snadwich Attack' (the need for more territory drives the increasing population of Snadwiches to be more agressive) which can often be fatal.
When a human sees a Snadwich, very often they mistake it for a ball of dust or hair.
Generally achieved while flaccid, unless the snadwich eater is particularly skilled.
Bonus points for using different person's testicles.
His dick was so small I could give him a snadwich easy.