What is hidden in 3D image?

An elderly couple who were childhood sweethearts had married & settled down in their old neighborhood.
To celebrate their 50th wedding anniversary they walk down to their old school. There, they hold hands as they find the desk they shared & where he had carved "I love you, Sally".
On their way back home, a bag of money falls out of an armoured car practically at their feet. She quickly picks it up, & they don't know what to do with it so they take it home. There, she counts the money, & its fifty-thousand dollars.
The husband says: "We've got to give it back".
She says, "Finders keepers" & puts the money back in the bag & hides it up in their attic.
The next day, two policemen are going from door-to-door in the neighbourhood looking for the money show up at their home.
One knocks on the door & says: "Pardon me, but did either of you find any money that fell out of an armoured car yesterday?"
She says: "No"..
The husband says: "She's lying. She hid it up in the attic."
She says: "Don't believe him, he's getting senile." 

But the policemen sit the man down & begin to question him.
One says: "Tell us the story from the beginning."
The old man says: "Well, when Sally & I were walking home from school yesterday ..."
At this, the policeman looks at his partner & says: "We're outta here ..."