The Olympic and Paralympic 'Blade Runner' denies the premeditated murder of his girlfriend Reeva Steenkamp on Valentine's Day last year, saying he shot her in a tragic accident after mistaking her for a night-time intruder.
On day 10 of the trial, Colonel Schoombie Van Rensburg, the first policeman to arrive at Pistorius' home in an upmarket Pretoria estate, expressed his anger at forensic blunders but had his own work called into question by the defense.
Van Rensburg said the missing watch was one of eight found in the house.
"I told the whole forensic team that those watches were a big concern of mine," he told the court. "Then one of the forensic experts mentioned that this particular watch, a green and black one, was valued between 50,000 and 100,000 rand (£2,800 to £5,600).
"I went down to the garage, then one of the officers came and mentioned that one of these watches is missing. And I said 'What do you mean? I can't believe it. We were just there.'"
He said everyone in the house, including all of the police officers present, were then subjected to a body search and their cars were also swept, but the watch had vanished.
In another example of botched police work, Van Rensburg described how a ballistics expert had held Pistorius' gun in his hands without gloves. "I was very angry and was not very pleased with what I saw," he said.
This may not be material to the case as there is no dispute that Pistorius fired the fatal shots. But the overall credibility of the investigation could still prove crucial to the outcome of a trial that has gripped South Africa and is expected to last for several more weeks.
Days after the killing of Steenkamp last year, it emerged that Hilton Botha, the initial lead detective in the case, was himself facing attempted murder charges for firing on a minibus full of passengers.
Ridiculed for his slipshod handling of the initial investigation, he was pulled off the case and then resigned from the force. Van Rensburg has also subsequently retired but remains a key witness.
If found guilty of premeditated murder, Pistorius faces at least 25 years behind bars. The athlete says he was convinced he was shooting at an intruder when he fired four bullets through a locked toilet door, three of which hit Steenkamp.
The first photographs of Pistorius after the shooting were displayed in court on Friday, showing the heavily muscled athlete bare-chested and staring vacantly at the camera, with blood spattered on his rumpled shorts and left arm.
Pistorius had his lower legs amputated as a baby, but he overcame the disability to become the "fastest man on no legs", running on carbon-fiber "blades" to win gold medals at the Beijing and London Paralympics.
He was not wearing his artificial legs at the time of the shooting, and has said this made him feel vulnerable and panicky, part of the reasoning behind his plea of 'not guilty' to the murder of law graduate and model Steenkamp.
His lawyer Barry Roux tried to raise questions about the police handling of the crime scene, noting that in one police photo a cell phone was visible, whereas Van Rensburg conceded it was covered by a towel when he first arrived.
"We are trying to find out when can we rely on a scene photo as a true reflection, and when is it changed," Roux said.
The trial resumes on Monday.
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