"We sat for five minutes with our jaw dropped," she later told news channel WTHR. "It's like something out of a movie."
But when she tried to finally get in touch with her long lost mother, she said Ms Gillespie-Miller wanted nothing to do with her.
A phone call to her mother on Friday last barely two minutes before Ms Gillespie-Miller told her she could not talk.
"She said, 'I'll call you when I'm able to talk,'" said Ms Miller, but it is a phone call she believes will never come.
"I will never call her again," she told PEOPLE. "It felt like being rejected all over again."
Ms Gillespie-Miller had just given birth to her third child and signed responsibility over to her parents when she disappeared.
She told police this week that she "felt she was too young to be a mother at the time", NBC News reported.
And so she stepped through a door and out of the family's life. Other than a letter from 1975 with a Richmond, Indiana, postmark, they had no idea where she was or whether she was safe.
But they never stopped hoping; Emma Gillespie, Lula's mother, believed she might see her daughter again right up until her death at 91.
"She always left her porch light on every night because she always thought that Lula was going to come home. She never stopped doing it," Ms Miller said.
For Ms Miller, who was two when her mother left, it led to many painful years of wondering. "You think 40 years, you haven't heard anything by now, the chances are slim," she told Eyewitness News in 2014.
"You sit and you analyze the picture. Do I have her eyes? Do I have her cheekbones? Do I look like her at all?"
Ms Gillespie-Miller's name was registered with The Doe Network, an international center for missing and unidentified people, and it was that organization that got the investigation going again, four decades on.
In January 2014, Indiana State Police Detective Sergeant Scott Jarvis picked up the cold case, and The Doe Network reached out to him.
Following the trail set by the 1975 letter, he contacted the Richmond police, who said they had a case of an unidentified woman found dead in 1975.
Sergeant Jarvis had the body exhumed for DNA testing in December that year. He told the Pal-Item at the time: "The daughters I'm in touch with are excited. They were two or three years old when their mom left.
"They're excited about the possibility of finding her for closure, but they don't have their hopes up. It's still just a possibility right now."
Of course, that body turned out not to be Ms Gillespie-Miller, but Sergeant Jarvis picked up another thread: a woman fitting her description who lived in Tennessee in the 1980s, then moved to Texas.
On Thursday he called the Texas Rangers and asked them to go to the woman's home.
When asked by officers, the 69-year-old woman admitted that she was Lula Gillespie-Miller.
As she never actually committed a crime, police say, she has the right to anonymity - although she has given permission for them to give her address to Ms Miller, so that the two can be reunited.
But Ms Miller said that two-minute phone call brought a whole new feeling of grief and pain as she was forced to deal with the fact that her mother chose to abandon her.
"It's almost like going through the grieving process again," Ms Miller said. 'I'm glad she's alive, but it hurts emotionally knowing this was her choice."
- Daily Mail
