Joseph Jackson said Muir turned up at his and his mum’s home five days after she went missing, claiming she had gone away after Botox injections went wrong.
Joseph said Muir’s account didn’t add up and he became anxious and contacted his brother, who rang police.
The 23-year-old said Muir told him he had last seen Pamela on Saturday, March 2 but then when police turned up at the house soon afterwards he said it was Monday, March 4.
Prosecutors claim Muir murdered 55-year-old grandmother Pamela at her home in Chester-le-Street, County Durham, on March 2 then buried her body on bleak moorland 120 miles away near Halifax, along with flowers in a plastic bag. Her body was not found for nearly two months.
Joseph told jurors at Newcastle Crown Court: “He told me before the police arrived he last saw her on the Saturday.
“He told the police he saw my mother on the Monday and that was the last time he saw her.
“When I heard that I was like, “whoa, that’s not what you told me”. I’m crystal clear that’s what he said.
“Then half an hour later when it was fresh in my memory he told the police a completely different account.”
Joseph, a university student who was working part-time at Wilkinson’s, said Muir’s story about the Botox didn’t add up.
Muir claimed Pamela had asked him to inject him with £3,000 of Botox she had got off a friend but said he refused because he didn’t like the sight of blood.
He claimed Pamela then injected herself with it and it made her skin wrinkly. Muir claims there was then an argument and she decided to go and stay with a friend.
Joseph said: “He said she had bought some Botox needles from a friend who is a chemist. That puzzled me because she didn’t have any friends I know that was a chemist.
“He said she had gone to stay with a friend for a while and again I was puzzled because there’s no one I can think of she would go and stay with and has never done it before.”
Joseph said he went to work that night and when he returned Muir was playing guitar and had bought lots of Easter eggs for the family. Joseph said he was becoming increasingly worried about where his mother was and said Muir’s story “barely added up at all”.
He spoke to his brothers on the phone and one of them came round and the other phoned the police.
Joseph said: “I asked Adrian about where the needles for the Botox were. He said he didn’t know.
“I went to check the bathroom and he said they might be in a leopard skin handbag in the living room. There were no needles anywhere.”
Joseph also said he had noticed a wet patch on the carpet in the living room that same night.
He said when the police turned up Muir moved where he was sitting and so he was leaning over the wet patch.
The police and Joseph’s brother eventually left him alone in the house with Muir.
He got up and went to university the next day and when he got home the house was cordoned off and the police investigation had begun.
Muir, 50, of Calder Terrace, Halifax, a stonemason, denies murder.
His barrister, Robert Woodcock QC, said: “The defendant’s case is she was in the habit of meeting other individuals, one of whom must have been the person with whom she disappeared with on March 2.”
The trial continues.