Airport security staff spotted another small animal inside a U.S. traveler’s checked bag this week. While screening luggage as it passed through the X-ray machine at Dane County Regional Airport in Madison, Wisconsin, agents discovered a dog packed into what appeared to be a backpack small enough to fit in one of the security trays, according to the Transportation Security Administration

The agency, which often recounts startling finds by TSA agents working security stations across the country, said the pet “was accidentally sent through” the machine in a message posted on Twitter. The passenger was unaware of screening protocol and did not alert the Transportation Security officers that the small dog — a dachshund/Chihuahua mix — was in a backpack-style carrier, a TSA spokesperson told CBS News. The officer confirmed the passenger had told the airline she was traveling with the dog, and she headed to the gate after her bags were cleared, the TSA spokesperson said.

A tweet from the TSA Great Lakes branch shared Tuesday included side-by-side images of the dog as it appeared on the airport security scanner, and the backpack where it was found.

“When traveling with any animal, notify your airline & know their rules. At the checkpoint, remove your pet from the bag and send all items, including the empty carrier, to be screened in the machine,” the tweet sid. The agency also shared a video of an airline passenger traveling with a cat to demonstrate the correct procedure.

“Here’s the proper way to travel with your pet,” wrote TSA Great Lakes. “Note: This is a @TSA PreCheck passenger traveling with a cat. If you think your pet will attempt an escape, ask to speak with a supervisor before removing the animal. Alternative screening options may be available.”

Individual airlines have different protocols for passengers looking to travel with pets. But, typically, most require anyone traveling with a small animal to bring it along in a carrier small enough to remain with the traveler for the duration of their flight. Like the cat owner in TSA’s social media video, travelers must be able to remove the pet from the carrier and send the empty bag through the airport’s security checkpoint for screening. 

The rules are slightly different for service animals, but no scenarios exist where airline passengers are permitted to travel with unreported pets inside checked baggage.