The doubtful detective

Ernie longed for a day where he would outsmart Jack, but he didn’t know what to do. One day, he had a plan. The next day, each kid received a card, slipped under their doors.
Ernie Tannen, Private Detective
Prices at 5p
Location: Picksbridge Park
As Jack picked it up, he sensed something different.
“Hey there!” Jack looked up. It was Jim. “Anything wrong over there?” As Jack explained, Jim nodded his head sympathetically.
“Yeah, I hear he already has a case. Albert’s coin collection’s missing.”
“I bet he stole it.” Jack said bitterly.
An hour later, he saw Ernie with a big grin.
“There’s a new detective in town!” he crowed, brandishing the collection of coins.
“I was out searching when I found this cat, and I advanced on it. It hissed at me, and I brought out my sandwich, so I could give it my meal. It was wrapped in aluminium foil though, so the cat was scared and ran away. I followed it, and I saw something shiny. And it turned out to be his coin collection.”
Ernie smiled, and a grateful Albert took the collection back.
“Stop!” said Jack. “Ernie is lying!”
HOW?