Mr Singh filmed his wife rescuing the woman in case anyone accused them of breaking into the shop. "She spoke to my wife and I recorded everything."
It took her a minute before the door swung open.
The distraught staff member broke down in tears and wrapped her arms around Mrs Singh.
The woman, who appeared to be in her early 20s, thanked the couple over and over again after being locked out for almost two hours.
Mr Singh said she was shivering and visibly upset.
"She was very, very frightened. She was out of her mind. She was saying, 'you guys are angels'."
The couple offered to give her a ride home, but she refused saying she had to close up and lived close by. They stayed with her for five minutes to check she was okay.
He said the worker had also been worried about the shop being left unattended for several hours while she was locked out. The store had also been due to be closed at 9pm.
There was no one around at that time of night and Mr Singh wanted Subway to review their policies around staff working alone as he believed it was unsafe. He said they had been to Mission Bay and had decided to stop for a snack before heading home.
Subway communications manager Ben Miles confirmed the worker had gone out the back of the shop at 8pm to use a communal toilet -- not a chiller as the Singhs had believed -- and forgot the security code to let her back in the firewall.
No one was at the other shops which shared the toilet. "She didn't have her phone on her and I think she's been a bit embarrassed about the whole thing."
The worker was fine and would be back at work on Wednesday, he said.
Mr Miles said it was not uncommon for staff members to be left in sole charge if it was safe and reasonable to do so.
"We base our staffing based on the sales. Being an adult and fully qualified, we are able to have one person on just like an other convenience store or small business which needs to manage its costs."
