Search continues for missing grandfather, 65, with dementia who was last seen hitchhiking on a Perth highway three days ago after leaving his home to go for a walk

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A desperate search is under way for a grandfather-of-ten with dementia who hasn't been seen for three days.

Ian Collett, 65, left his home in Canning Vale, Perth, about midday on Saturday.

His panicked wife Deborah said she believes he went for a walk but got disorientated. 

Ian Collett, 65, has been missing from home in Canning Vale, Perth since midday on Saturday

Mrs Collett (centre), who went for a walk to find her husband with no luck, said he is a fit man who walks two or three times a day

Three hours later, the father-of-four was picked up by security cameras at a 7/11 sipping a McDonald's drink and wearing a coat that wasn't his.

Nobody knows how he got to the convenience store more than 20km from home. 

Mrs Collett, who went for a walk to find her husband with no luck, said he is a fit man who walks two or three times a day.

But she fears his feet will be very sore and said he struggles to express pain.

Speaking at a news conference, Mrs Collett said her husband is wearing an alert bracelet with the word 'dementia' written on it along with her contact details.

'We just want him back – we miss him,' she said. 

The search team includes 40 police and Mounted officers and Airwing officers, along with 50 SES personnel.  

Crews focused their search for the grandfather in Armadale, particularly along Albany Highway and police are requesting drivers who passed the area at 5.53pm on Sunday to check their dashcam footage.

Police are urging people in Byford, where Mr Collett was least seen on Saturday at 4.30pm, to check their properties and their sheds to see if he is resting there.

Mrs Collett (left) said her husband is wearing an alert bracelet with the word 'dementia' written on it along with her contact details

Police are urging people in Byford, where Mr Collett was least seen on Saturday at 4.30pm, to check their properties and their sheds to see if he is resting there. Pictured: Search teams