The Ice cream

After these events, Jack decided he would open a detective agency, charging only 10p. That evening he got his first customer. “I want to hire you,” said Henry, a local boy.
“Why?” asked Jack.
“Well, it’s just-“
“Murder?”
“Erm. No, it’s-”
“Blackmail? Kidnapping? Art Forgery? Money Laundering? Fraud?”
“No!”
He explained.
“I run an ice cream stand. Since it’s hot today, about thirty degrees, I went to run my stand. But Ernie came and stole an ice cream cone off me.”
“Do you have any money to hire me?”
“No, but I do have an ice cream.” He thought it over.
“How much does he owe you?”
“50p.”
“Where would he be?”
“In his den, in that shed at the park.”
They left for the park.
Ernie greeted the detective by saying: “Get lost.”
Jack responded by saying: “My client, Henry here, says you stole the ice cream you’re eating, off his stand.”
“What? I never met little entrepreneur here. I got his ice cream from the store this morning. It came in a packet of four.” said Ernie.
Jack want down on all fours and searched the floor.
“Found anything?” sneered Ernie.
“Nothing.” replied Jack. “And that proves it!”
Why?