Police in Markham were "quite concerned" last night about the sudden disappearance of 25-year-old Alicia Ross, but had not deemed the case foul play.
Ms. Ross, who works at a nearby computer firm in Markham, was reported missing on Wednesday.
It is unlike Ms. Ross to simply disappear without telling her loved ones, police say.
Ms. Ross left behind her purse, car keys and money. Her car is parked outside the suburban Markham home where she lives with her parents and siblings.
She was last seen by her boyfriend wearing a yellow tank top and baggy grey pants outside her home at about midnight on Tuesday.
For at least seven hours yesterday, police searched a 300-metre-radius grid in the steep, heavily wooded ravine behind Ms. Ross's home. Canine units were there, a mobile command centre was parked in the driveway and officers had blocked off the road leading into Ms. Ross's street.
The search was halted after darkness fell, but is to resume this morning over an area twice the size of yesterday's grid.
Ms. Ross had failed to show up for work at Hewlett Packard on Wednesday.
"It's an out-of-character incident at this point, and we are seeking the public's assistance," said Laurie Perks, media officer for York Regional Police.
On Wednesday, a neighbour said she saw six police cars parked outside Ms. Ross's family home in a well-to-do neighbourhood.
Her father was pacing in the middle of the roadway, the neighbour said.
By yesterday afternoon, media had taken over the street, two helicopters circled overhead and friends and family had gathered outside the quaint brown-brick home.
A rabbi entered the home at around suppertime to pray with the family and hang a Jewish prayer scroll on the young woman's bedroom door.
"It's supposed to be for safety and good omens," said Rabbi Abraham Plotkin. The family is "distraught," he said.
Mr. Plotkin said he has e-mailed residents in the neighbourhood, asking them to show up en mass to search for Ms. Ross.
Ms. Perks asked neighbours walking dogs in the area to notify police if they find anything suspicious.
Ms. Ross's boyfriend, Sean Hine, told CTV that police had questioned him and searched his car and home. In an interview, he produced photos of the couple on vacation and said: "I loved her."
York Regional Police's homicide squad has been called in to help with the search, Ms. Perks said.
Detective Constable Daniel Farewell of Toronto's 52 Division said about half of the people reported missing in Toronto turn up within 24 hours.
Almost all of the rest turn up within two to three days, he said.
Most have either runaway, or are elderly people dealing with Alzheimer's or memory loss.
York Regional Police do not keep statistics on how many people are reported missing in a year, Ms. Perks said.
Detective Brad Hooper of the Toronto Sex Crimes Unit said that the level of search police will conduct depends on the individual circumstances of the case. They look at the age of the missing person, how well that person is able to take care of himself or herself and for any signs of foul play.
Ms. Ross's parents and relatives did not wish to speak to the media, which is not unusual, according to Bobbie McMurrich, program manager for Toronto Police victim services. She and her department deal with dozens of missing-persons cases every year and said Ms. Ross's parents must be extremely anxious.
"Because it's so out of character for her to not be coming home, her parents must be extremely fearful of her safety," she said.
"They're probably at home, waiting and waiting and wringing their hands. . . .There's no way they'd be able to sleep."