"You spend your career looking for something you don't want to find."

SPOKEN BY A TRAINER, those words are burned into the brain of Officer James Cassidy, a four-year member of the elite Philadelphia Police Airport Explosives Detection K9 Unit. The cops have a single goal - to protect travelers by detecting explosives.

Each cop's partner is a highly trained dog that gets the last word whenever explosives are suspected. Cops are taught: "Trust your dog."

"If he says no, it's no," says Officer Terry Wright, 41.

The way an airport K9 dog says "no" is to sniff something and move along. An airport K9 dog says "yes" by sniffing, then quickly sitting down.

In a real-life situation, when a dog sits it means trouble. The area - whether baggage, concourse or terminal - will be cleared and locked down in minutes just because the dog said so.

That's never happened at Philadelphia International in real life, although K9s sit every single day in training sessions. Dogs detect explosive odors planted for them to find.

As part of that training, some days K9s are taken to the Hertz car lot to check parked autos for explosives. Other days, scents are hidden in baggage areas or on aircraft.

For security reasons, Airport Capt. Denise Turpin won't enumerate the odors her dogs can detect, but they are able to add to their repertoire as new threats emerge, according to TSA spokesman Greg Soule. FBI agents Mike Baker and Dale Carlson help keep the dogs up-to-date on new scents.

"For them, it's hide-and-seek. It's a game. We hide something, they find it," says Cassidy, 47, whose partner is 6-year-old Britt, a 90-pound German Shepherd with a dark muzzle and a bushy tail.

The game reinforces dogs' training, and handlers say dogs would become frustrated if they never found anything. A "hit" is rewarded by verbal and physical praise, plus a short play session with a Kong hard-rubber toy. They are just crazy for the Kong.

Dogs are bought and trained by the Transportation Security Administration, but once assigned to an officer they remain with that handler on and off the job.

Rex, a 7-year-old, 77-pound Belgian Malinois with a graying muzzle and pointy ears almost as big as pizza slices, has been Wright's partner for the five years he has been a handler.

"This will be his last year," Wright says. "He's worked a number of years. He's done a good job."

When Rex retires, Wright gets to keep him at home, which is the right thing to do for the tightly bonded team.

As Wright explains it, going to work is play for Rex, who is active and involved with his partner all day. "Play time is here," Wright says, indicating Terminal C, where we are standing, before Rex checks out passengers and baggage with his peerless nose. Wright rarely plays with Rex at home.

Cassidy, conversely, does play with Britt in his Northeast Philly home, where Britt sleeps at the foot of his bed.

Each handler does it a little differently because each dog is different.

 During each shift, K9 cops check the airport's perimeter fence for unlocked gates, breaks in the fence, or cars that don't belong there. K9 teams check the cargo building, walk through public areas to provide a reassuring presence for passengers. Daily chores are done randomly to avoid creating a terrorist-helping routine, Wright says.

Airport coverage is 24/7, but Turpin declines to provide the precise number of teams. In January, in news stories reporting on three TSA dogs that had flunked certification, the number of teams was placed at more than a dozen.

Philadelphia police-dog trainer Les Johnson pointedly notes that those failing certification were TSA handlers, not Philly Airport cops.

Johnson, 49, is teamed with a 57-pound, 3-year-old yellow Lab named Tucker who can almost pull the 230-pound cop off his feet. "You don't want to correct them when they pull" following a scent, says Johnson, although "90 percent of the job is checking unattended luggage."

Around 11 a.m., Wright's driving his Ford Explorer around the airport. He pulls over suddenly when he spots a blue overnight case on the sidewalk.

He gets Rex out and they approach the case. When Rex shows no interest, Wright knows it's safe. Trusting his partner, Wright puts the carelessly discarded case in the rear of his truck.

Cassidy and Britt are in the secure passenger area, beyond the TSA checkpoint, where you can walk from Terminal A to E and back - and they do.

A former Marine whose cell ringtone is the "Marines' Hymn," Cassidy is Ninja-like - black boots, black pants, black shirt, black windbreaker. He's holding Britt's leash - always in the left hand - and weighed down with cop paraphernalia - handcuffs, radio, gun.

Most passengers seem to take no notice of the team, but most who do smile, with some mouthing the words, "What a beautiful dog." Few try to pet Britt, having the sense not to bother a working dog.

On breaks, dogs are turned loose in pet ports, fenced-in areas adjacent to each terminal, to relieve themselves.

The personable Wright - airport workers are always waving at him - eats lunch with Rex. "I eat in the Explorer so we can eat together," Wright says with a smile. "I have someone to talk to."

Cassidy also usually spends his lunch break with his furry partner.

There's no question about the affection between cop and dog.

Rex looks at Wright the way a Sorbonne art student looks at the "Mona Lisa." The trust is reciprocal.

"They're trained to be independent," Cassidy says. "If someone's walking by in the opposite direction and he sniffs something," Britt will turn and pull, and Cassidy will go with his dog.

"You put your life in his hands. If he says the area's clear, it's clear," Cassidy says, but "if a dog sits, that's it."

In the arrivals area, Britt pulls Cassidy to the carousel to sniff some luggage. No hits now - but there will be later.

Walking through the concourse, Britt seems blase, but his magic nose is constantly working, able to detect minute traces of explosives. Along the perimeter, he checks fire extinguishers, plants, telephones. If he misses something, Cassidy points with his right hand and Britt inspects.

Handlers study the dogs' behavior and body language.

When Rex's tail quivers, "he's on an explosive odor," Wright says. When Rex's tail is straight, he's on a "novel" odor, maybe one that he hasn't smelled before, but "he's interested."

Rex was interested in a tall, thin man checking in at Terminal C.

Rex put his nose on a carry-on and stopped. Rex was "interested," but did not sit.

Wright asked the man to open the luggage. The man looked annoyed, but unzipped a compartment. Rex's nose went in, followed by Wright's hand.

The scent came from Flomax, a prostate medicine.

The dogs frequently pay attention to medicine scents.

Since Rex cannot speak for himself, Wright sometimes provides a voice for his partner, such as after the Flomax stop: "Smells something like it, but that's not it, Daddy, so let's go."

Cassidy does the same thing: "He'll go, 'Yo, Dad, I got something here.' "

At the baggage carousel later, Britt makes several hits, finding explosive odors planted in a red suitcase by Ed McGinley, who works with a 9 1/2-year-old, 75-pound German Shepherd named Dago. The name is German, it's pronounced DAH-go and the dogs come pre-named, McGinley takes pains to explain.

While McGinley, 46, was planting scents, Dago hit a dashboard button in the Explorer, setting off the siren. An embarrassed McGinley ran outside to turn it off. Because the dogs don't all get along, they're sometimes left in the Explorer for short periods.

At shift's end, Cassidy tells me, "It's the best job in the department. I get to play with a dog and I get paid for it.

"One downside - I'm looking for explosives," something he doesn't want to find.

E-mail stubyko@phillynews.com or call 215-854-5977. For recent columns: