Police have spent the day with the family of Ian Collett after road workers discovered the body of a man believed to be the missing grandfather in bushland along Albany Highway in Ashendon.

Mr Collett, who had dementia, went missing from his Canning Vale home in May and despite a search covering nearly 10,000ha over 17 days, no trace of him was found.

The body was discovered in scrub about four metres from the road about 9.30am on Thursday.

Sadly, it was in one of the areas previously scoured.

Acting Commander Brad Sorrell said the search by police and volunteers was comprehensive and he was satisfied everything possible had been done to locate Mr Collett.

Police have not indicated how long his body had been lying there.

Albany Highway was closed for most of the day from Canning Dam Road to Jarrahdale Road while police officers examined the scene but has now been reopened.

Police have praised the efforts of those that assisted in the search, including SES volunteers and members of the public.