Animal rescue volunteers began a frantic search Monday night for a 10-pound (4.5-kilogram) dog named Emily who ran away from Toronto airport Terminal 3 after customs staff let her out of her crate.

The tiny dog — and new mother to a litter of puppies — was among a group of five who arrived from Athens with the local organization Stray Paws from Greece.

"We have no idea what's going on," the group's founder Anastasia Tasoula Somers said, about four hours after Emily went missing. "None of the people responsible for looking after the dog have come to say 'we're sorry' or tell us what's going on, but we're doing our best to find Emily."

Emily is about 18 months old and recently had a litter of puppies. Airport staff had not been able to find her as of 11:30 p.m. Monday. (Submitted: Stray Paws from Greece)

Canada Border Security agents held the dogs for three hours after their flight landed at 3:30 p.m. ET, Somers said.

Border officials spent that time questioning the escort who flew with the dogs, she said. After that, they told her that the dogs would have to be processed through commercial customs — something she alleges she's never been told to do in the years she's been running the group.

That meant the animals would be in their crates for at least another hour.

Somers said she asked CBSA staff if the group's volunteers could let the dogs out of their crates, but said she was told they could not.

'Such a little girl'

Instead, customs staff themselves let the five dogs outside — and Emily bolted. The area in which she went missing is a patch of grass and concrete behind Terminal 3, bordered by a parking lot and the highway, but not fenced in.

"I thought I would die," Somers said. "I just felt so bad. She's just such a little girl."

No one from CBSA at Toronto airport could be reached Monday night.

A spokesperson with the Greater Toronto Airport Authority, however, confirmed that airport staff were out looking for the dog, which had been in the care of the border agency.

Volunteers with Stray Paws from Greece said Emily was likely stressed from her flight, which is why she ran off. (Submitted: Stray Paws from Greece)

'It's been pretty heartbreaking'

The Richmond Hill family that had adopted Emily was still at the terminal at 11 p.m., hoping for a glimpse of the pooch whose smile had won them over.

"From the pictures that we saw, she's just a really happy dog," Jordan Wong said. "We've just been looking out the window and hoping we see her.

"It's been pretty heartbreaking."

'She could be out there looking for her babies.' - Jordan Wong, Emily's new owner

Wong, 20, had been at the airport with his sister and his parents since 3 p.m., expecting to meet the new addition to their family. They waited with four other families, none of whom knew, at first, which dog had been lost.

"We all just found out that a dog was missing, and it caused a lot of unhappiness in our group," he said. "But when we learned it was Emily, the other families didn't feel a sense of relief, because they knew that there was still someone's dog out there missing."

But Emily's a tough little pup, he said.

"She had a whole litter of babies, and this little 10-pound dog managed to keep all of the babies alive," he said. "So who knows what she's doing now? She could be out there looking for her babies.

"All we can do is hope and be patient."