The adjective shadowy is good for describing things that are mysterious or hard to see, like a dim figure in the fog or the social rules of a typical middle school.
Indistinct or faint objects are shadowy, whether they're distant people standing in a dark alley or birds you can barely see against the choppy surface of the ocean. You can also describe something that's ghostly or unreal as shadowy. A shadowy room or a shadowy figure brushing past you would both be insubstantial and vaguely spooky. The root of shadowy is the Old English word sceadu, "shade, shadow, or darkness."