DUMB PLOT HOLE (contains spolers):

Someone could have easily dug that pit themselves, given the time. No one's going to think "that one person has a hole in their backyard... We better go check there", especially if they have no real reason to look there.

No one probably knew it was there to begin with.

What the guy above said, plus…

1. There are no neighbors that close by.

2. The pit could've been there when they bought the house.

3. The pit could've been for something else, say, a septic tank or 50s fallout shelter, then just used by the lady when they had a need.

I was more wondering why no one ever seemed to check the vacant house where the RV was parked. At least to cover that possibility for plot purposes.

Not sure why this is considered a plot hole.

It could have been from something as simple as a sinkhole, an old sewer or other construction remnant, old well hole, cavity from an underground stump decomposition.

And if they were digging in the yard, they could just say it was to fix a septic tank or plumbing issue. I know people who have problems with their damaged sewer system and their yard is still torn up with holes in it from dealing with that.

I feel like it was just done because the director wanted to have the whistle scene as the ending.

Maybe you could masturbate less and actually dig a hole and see it's not that difficult.

I get it, it's a plot point about a hole, a plot hole if you will, but not the kind we're used to.

You answered your own question.
It showed how estranged they were from their neighbours!
It's how Josef Fritzl happened. Just google all the cases dubbed the "American Fritzl".

Indeed a strange thing: neighborly ties are dissolving, while little hives of religious nuts rise!

I beleive there are other unquestionable plot holes, way dumber than the ones you raise:

1) If one of the girls escaped, it is evident the police will soon be on your heels. Yet Holly did nothing about it.
2) If Alex disappeared after Keller all of Keller’s yelling to the police he was guilty, wouldn’t that raise suspicion. Only when the detective discovers Keller’s absence from home late at night does he decides to follow him, and foolishly gives himself away when he doesn’t pull over to let the garbage collector pass.
3) Keller discovers the police is following him, that he knows where he was heading but didn’t change his plans. Next, he is found in his secret quarters and was extremely close to be discovered, yet he did nothing about it