His girlfriend had never been uncontactable in their three-year relationship.
So when the 22-year-old undergraduate went silent on her mobile phone in the early hours of July 24, 2014, the worried boyfriend logged into her Apple account using their common password to search for her using the Find My iPhone app.
The boyfriend, now 28, was testifying in the High Court yesterday on Day 4 of the trial of Ong Soon Heng, 40, who also goes by the name Osh, for one charge each of abduction and rape.
The prosecution's case is that Ong had plied her with drinks, then drove her back to his home and raped her.
Yesterday, the boyfriend told the court that it was he who had encouraged his girlfriend, then an intern at a food and beverage company, to go to Zouk for a colleague's farewell party. Ong was a friend of the owners of the company.
She was hesitant to attend as she was tired but he persuaded her to go as a new co-worker was not acquainted with the rest.
When she stopped replying to his WhatsApp messages at about 3.15am, he assumed she was with her friends. But two hours later, she had not taken all seven of his calls.
Close to 6am, he logged into her Apple account and the Find My iPhone app showed her device was at Hume Heights.
He got into his van and followed the GPS to a carpark near five colonial black-and-white bungalows.
He recognised a Daihatsu parked there as Ong's car; his girlfriend had borrowed it when Ong was overseas. He knew that Ong, whom he has never met, was at the party so he inferred this was his home.
But he was "almost bitten by a dog" and chased out by the owner of the first house he tried, he said.
At about 6.30am, he spotted his girlfriend's shoes outside the door of one of the houses and knocked for five minutes before Ong's housemate opened the door.
The housemate became "panicky", glanced at a closed door inside the house and made a phone call that went unanswered, he said.
The boyfriend said the man let him into the house. He said he opened the room door, switched on the light and saw his girlfriend and Ong on a mattress, under a blanket.
He told the court Ong sprang up and said "I did not do anything". His girlfriend was wearing a T-shirt and shorts, he said, while her dress and belt were on a clothes hanger.
He said he kept shaking her very hard but she could barely open her eyes. He pressed Ong to explain what had happened but he did not get any answers.
He pulled her up to a sitting position before lifting her to a standing position. She was "extremely weak" and could barely stand, he said, and he had to "drag" her out of the house, putting her arm over his shoulder and his arm around her waist.
Ong's lawyer, Mr Peter Fernando, contended that the boyfriend's account was "entirely untrue".
The lawyer said his client's version is that the woman got up from the mattress on her own and walked out to the van unaided. He asserted that the woman then looked at Ong and stuck out her tongue "to indicate she was caught red-handed by you".
"I strongly disagree," said the boyfriend.
The woman, now 25, had testified behind closed doors on Wednesday.
A medical report said her blood alcohol concentration level at the time she was at Ong's house was between 210.50mg and 254.50mg per 100ml of blood. The legal limit for driving is 80mg per 100ml.
The trial continues.