About 90 minutes after starting to search for two 16-year-old girls who had been missing in Ontario's Algonquin Park since Thursday, Zoe began showing the telltale signs that she had picked up a scent.

"That's what you want to look for when the dogs acquire a track. There's a physical cue, or a difference in their behaviour," said Scott Gannon, an officer with the Ontario Provincial Police Canine Unit.

For Zoe, a two-and-a-half-year-old Labrador retriever, her nose will go down to the ground, she will start sniffing more, show interest in one area, wag her tail and get excited, Gannon said.

Gannon, Zoe, and two members of the Emergency Response Team made up one of the four search and rescue (SAR) dog teams that had been flown in on float planes to help search for Maya Mirota and Marta Malek. The teens had failed to meet up with their camping group in the provincial park's western sector.

The four teams were assigned to follow the hiking trail where the girls had last been seen. Search dogs are looking for a human scent, said Clifford Samson, a trainer in the OPP Canine Unit.

When the handlers are given a search area, they start downwind, and move the dog back and forth in hopes it will pick up the scent, Samson said.

During their search, "you could see where [Zoe] became excited," Gannon said.

Maya Mirota, left, and Marta Malek were found safe Monday in Algonquin Park. (CBC)

When Zoe hasn't picked up a scent, she stays close to Gannon. But once she gets that track, like she did on Monday, she will go much farther ahead and "she beelines it toward that source."

"She became excited and we watched her and then she proceeded to go down the trail," Gannon said.

With Zoe out ahead, Gannon and the crew began calling the names of the girls, who responded by blowing a whistle three times.

The police called for the girls to say their names.

"They gave us the correct names, because we wanted to make sure [it wasn't] someone else on the trail just hearing us yell," Gannon said.

"We told them to stay put and we followed the dog."

They found the girls in good physical condition, although a bit dehydrated and covered with insect bites, Gannon said.

"They were very excited and happy to see us, and we were the same way," Gannon said. "They both gave us hugs."

Of course, they were also excited to see Zoe, asking if they could pet their rescuer.

The girls had set up camp for three days, trying to get the attention of aircraft flying overhead. But they were not in an open area where they could be easily spotted, Gannon said.

"So today they decided to walk, and they ended up walking on to this trail and then we just happened to come up behind them using the dog."

When Zoe, a two-and-a-half-year-old Labrador, has picked up a scent, her nose will go down to the ground, she will start sniffing more, show interest in one particular area, wag her tail and get excited. (Scott Gannon)

Gannon said they walked the girls back about 1.5 kilometres, where a float plane arrived to take them back to the OPP command post.

The canine unit has 27 service dogs, 11 of which are dedicated to search and rescue missions, Samson said.

The handlers and their search and rescue dogs take a 10-week training course at one of the OPP's three training centres, he said.

Friendlier breeds of dogs

The SAR dogs are usually friendlier breeds of dogs, like Labradors, which have a good nose and a high drive to want to search, Samson said.

"Labs are notorious for being friendly and loving people. And that makes them a great dog for that job," he said.

Police also look for dogs that have a high "play drive" — the breed "really likes to chase a ball and wants to play," Samson said.

As part of the training, the instructor will send someone to hide with a ball. When the dog locates the person, it's trained to bark.

(CBC)

"We start off with the person that's hiding having the ball to make that person valuable for the dog to find," Samson said.

The person will then throw the ball, making it a game for the dog, Samson said.

"And we just work on that, make it more and more difficult and longer and longer searches."

The training then switches to the handler rewarding the dog, rather than the person the dog has found.

"Because somebody hiding in the bushes is not going to have a toy to reward your dog," Samson said.