At one point Marcus holds up a photo of a piece of evidence - a computer hard drive. On the label of that hard drive is a QR code. When scanned with a bar code scanner app it reveals the words "Hello World".
The title, "The Worms Crawl In, The Worms Crawl Out" is part of "The Hearse Song", a song popular with adults and children dating back to at least World War I. It goes as follows:
"Don't you ever laugh as the hearse goes by,
For you may be the next to die.
They wrap you up in big white sheets
and cover you from head to feet.
They put you in a big black box
And cover you with dirt and rocks.
All goes well for about a week,
Until your coffin begins to leak.
The worms crawl in, the worms crawl out,
The worms play pinochle on your snout,
They eat your eyes, they eat your nose,
They eat the jelly between your toes.
A big green worm with rolling eyes
Crawls in your stomach and out your sides.
Your stomach turns a slimy green,
And pus pours out like whipping cream.
You'll spread it on a slice of bread,
And that's what you eat when you are dead."
"Don't you ever laugh as the hearse goes by,
For you may be the next to die.
They wrap you up in big white sheets
and cover you from head to feet.
They put you in a big black box
And cover you with dirt and rocks.
All goes well for about a week,
Until your coffin begins to leak.
The worms crawl in, the worms crawl out,
The worms play pinochle on your snout,
They eat your eyes, they eat your nose,
They eat the jelly between your toes.
A big green worm with rolling eyes
Crawls in your stomach and out your sides.
Your stomach turns a slimy green,
And pus pours out like whipping cream.
You'll spread it on a slice of bread,
And that's what you eat when you are dead."
When Sherlock instructs Detective Bell at the shooting range, Marcus replies, "Don't worry; I'm not throwing away my shot". This is a reference to the song "My Shot" from the wildly popular Broadway musical "Hamilton".
The concept from the intro of a murderer using paint to hide in plain sight by blending into the wall is a nod to Stephen King's short story "The Doctor's Case", a pastiche where it turns out the killer hid behind a photorealistic painting of the actual wall behind him.