Chapter 1: The Note Inside the Mattress

My wife, son, and daughter never came back from their trip — 40 years later, I found a note that said, "If you're reading this, it's time for you to learn THE TRUTH."
My wife, Clara, took our daughter and son on a trip to visit her sister, who lived near the ocean.
I had a ton of work at the time, so they went without me.
But a few hours later, Clara's sister, Gwen, called me and said they had never made it to her house.
We even called the police.
Clara and the kids had simply vanished.
When the police began searching for them, all they found was their empty car abandoned on a bridge over a river.
The car was damaged.
The search went on for years.
Divers searched the river again and again.
The police had countless theories, but the most likely one was that the car had crashed on the bridge and the impact had thrown them into the water, where the current had carried them away.
Two years later, they were legally declared DEAD.
After that, I didn't know how to go on living.
I never even touched my son's or daughter's rooms.
It was as if part of me believed they would come back one day.
Forty years passed, and my heart still ached.
Then one day, my doctor told me that because of my health, I could no longer live alone and needed someone to care for me.
So my nephew said we had to sell my house and that I would move in with his family.
That meant I had no choice but to finally pack up the belongings of my wife, daughter, and son — things I HADN'T TOUCHED in all those 40 years.
While I was going through my daughter's room, I got so tired that I sank onto her bed and heard a crack.
I pressed my hand against the mattress and felt something HARD.
I flipped the mattress over and found a crudely stitched slit.
I grabbed a pair of scissors, cut it open, and pulled out a BOX that had been hidden inside the mattress.
Inside the box was a handwritten NOTE.
It was in Clara's handwriting.
The first words made my hands shake:
"If you're reading this, then something has happened to me.
It's time for you to hear the truth.
Don't trust Gwen.
There is SOMETHING you never knew."
By the SECOND LINE, I was sobbing.
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What did Clara know about Gwen that she h*d for 40 years?
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And why did she hide it where only he would find it?
Comment **"TRUTH"** if you want Part 2 — it's in the comments below.