Something that pops up a lot in the last few episodes is the question "How could you not know?"

This questions takes many forms and applies to almost everyone in the town:
"How could you not know" that your husband was having an affair?
"How could you not know" that the local newsagent and leader of a kids club was convicted for statutory rape?
"How could you not know" that your children were not best friends anymore?
"How could you not know" that your child had a secret mobile phone?
"How could you not know" that your young boy was sneaking off to meet an adult man during the night? 
"How could you not know" that your husband was sneaking off to meet a young boy during the night?
"How could you not know" that your husband killed Danny Lattimer?

Secrecy and lies are what tear this community apart, with the murder the culmination of everyone's failure to notice what is going on with those around them. Mark at one point said he felt "trapped", seeing the same dead-end routine of his past and future converge and laid in front of him for the rest of his life, inescapable. This life, with everyone stuck in the same small-town routine, breeds complacency and allows abuse and murder to happen right under everyone's noses. 

"How could you not know?" Well, it's simple: no one wanted to know, and no one bothered to look.