Other mistake: The Bundys never lock their front door the entire series, but a burglar has to use a crow bar to open the sliding glass door out back.
Revealing mistake: The burglar uses a crow bar to open the sliding glass door. You can plainly see him "pretending" he had to use it when it just opens easily. He didn't really pry it open.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.