Army, police bungled Sargent probe: Dad

Sydney Sargent did everything he could to try and find his missing son and at the time he thought military and police investigators were doing a wonderful job.

But after listening to days of evidence at a coronial inquest into the March 1999 disappearance of army Lieutenant Sean Sargent, he has come to a different realisation.

"It just wasn't handled correctly," he told Brisbane Coroners Court.

"If I didn't keep stirring the pot and grasping at straws, nothing would have happened."

Mr Sargent told the inquest on Wednesday he had contacted investigators at least once a week and called his son's phone thousands of times.

He doubts the army would have done much to find his son were it not for lead military investigator Army Reserve Sergeant Thomas Bere.

When first told his son was AWL (absent without leave), Mr Sargent feared the worst.

"I've always thought the worst but every time I've thought the worst, I've felt guilty, so I grasped at the next straw floating past, for 18 years," he said.

"I've tried everything."

For a while, he hoped his son, an avowed fan of James Bond and spy novels, was still alive and had changed his identity.

"But eventually you work out that it doesn't make sense," he said.

Mr Sargent told Coroner John Hutton he believed because his son's car also vanished, he was likely in it when it disappeared.

"You can't push a car into a river, you can't push a car over a bank ... you have to be with it," he said.

Earlier on Wednesday, Rachel Kayrooz, who dated Sean for about five weeks, described him as the "perfect boyfriend".

She rejected rumours he may have been hiding his homosexuality from the world.

Ms Kayrooz didn't believe he was still alive.

Sean's sister, Nancy Anderson, said she immediately thought he was dead when he didn't arrive for duty because he was "regimented with his work" and it was extremely out of character.

"I actually went into full-blown panic because that was nuts for him," she said, breaking down in tears.

"My gut feeling to this day was that (he was dead) but hope makes you push through because if you let go of hope, nobody looks for him."

Mr Hutton has indicated he is likely to find that Sean Sargent died in unknown circumstances.

The inquest continues.