The secret shed is the hideout of Red Herring.
A Pup Named Scooby-Doo
The gang followed tire tracks from Hedda Herring's missing motorcycle to the shed. They found the bike with a sidecar attached to it. The Boogey Biker attacks the gang, but Scooby accidentally turned on the bike, and crashed into the Biker. The Biker was unmasked as Red Herring, who didn't steal the bike, but borrowed it to put a sidecar on it as a Birthday present, so Hedda will have someplace to carry her groceries.