To be more specifically, the thing I'm thinking about is the whole sunglasses stigma.
Shino isn't the first mysterious guy who wears sunglasses in a manga/anime.
In Blade of the Immortal you have Giichi, a person part of an certain ambiguous group who has a rather *interesting* weapon he uses.
here's a link to his description:
Giichi
SAME SUNGLASSES! ... ok so he's bald, but then there's Mido Ban of Getbackers, Vash the Stampede from trigun, and the list goes on. Every one of em can be considered a mysterious character... is that what sunglasses can relate to? I suppose you can think of sunglasses in the metaphorical sense as something covering the eyes... the eyes being what describes a person the most or something...
(edit: changed the pic to a link to appease the internet gods)