1:30PM FRIDAY: There appears to be a new development in the case of missing Melbourne millionaire Herman Rockefeller - but police are refusing to confirm any details.

Victoria Police have only told 3AW Radio they are following up a number of new leads.

As a result of those leads investigators are on their way to Heathcote in Central Victoria.

Police say they are unable to reveal any other details because of operational reasons.

A 57-year-old man and a 41-year-old woman were arrested overnight and a townhouse was searched at Hadfield in Melbourne's northern suburbs.

Stay tuned to 3AW by listening live now or check back in with the website regularly for the details of the development.

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POLICE UPDATE: Police have executed a search warrant at a house in Hadfield in relation to the disappearance of Herman Rockefeller.

Homicide Squad detectives, with the assistance of Stonington Crime Investigation Unit, raided the South Street address at around 4.30pm on Thursday.

A 57-year-old man and 41-year-old woman were taken into custody. The Hadfield pair are currently assisting police with their enquiries and no charges have been laid.

Detectives worked through the night and are yet to establish a motive for Mr Rockefeller's disappearance, however, Homicide Squad detectives confirm they hold 'grave fears' for Rockefeller's welfare and investigations are continuing.

Neil Mitchell says the above information is simpy translated as 'police believe Rockefeller has been murdered'.

REPORT (John Silvester and Paul Millar): The search for missing businessman Herman Rockefeller yesterday took an ominous turn with detectives receiving information indicating he may be the victim of foul play.

Police yesterday raided a townhouse in Hadfield. Neighbours said a couple were driven away by detectives. No charges have been laid.

Forensic experts and detectives last night were searching the home for evidence to establish if Mr Rockefeller had been to the South Street address in Melbourne's north. They were also asking neighbours if they had seen the missing man or his vehicle on the night he disappeared. Forensic police were seen entering the townhouse last night. Police guarding the scene refused to comment.

It is believed they have established a link between the missing man and the couple.

In the past 24 hours the mystery has shifted from a missing person's case to a potential homicide investigation. In a new development a Sydney man has come forward claiming he received text messages and calls sent to the missing man's old mobile phone.

Police confirmed yesterday they were studying calls made to a phone, now owned by a Sydney man, who claims he had been receiving private calls for a man called Herman for the past 12 months.

The owner of the telephone said that he had bought the mobile about a year ago. "Over the past 12 months I've had some calls and they've asked for Herman," said the man, who would only identify himself as Mark.

"It's obviously interested the police. They said they will send somebody around to download all the stuff," he said, adding that some of the messages were personal.

While several theories are being investigated concerns are mounting for the safety of Mr Rockefeller, 52, who has not been officially sighted since he returned eight days ago from a business trip to Newcastle. His wife, Victoria, was at the Australian Open on the night he flew in from Newcastle.

He left the long-term car park at Melbourne Airport but he did not travel the usual freeway route to his East Malvern home.

He has made no contact since, has not accessed his credit cards and not activated his mobile phone. His blue Prius sedan was found dumped at Ballan near the Werribee River.

The Hadfield townhouse is less than 15 minutes from the airport.

One theory under investigation is the the hard-working multimillionaire may have voluntarily gone to an address on his way home. What then occurred is unknown.

Police initially believed Mr Rockefeller may have disappeared voluntarily and might have had a mental breakdown. They have established he was not the victim of a carjacking, have no evidence of a kidnapping and nothing to indicate he was an extortion or robbery victim.

They are increasingly convinced there must have been some third party involvement and their hopes he voluntarily absconded and is in hiding are fading.

His brother, Robert, who was with him on the four-day Newcastle business trip, said he was in good spirits when he left.

With BEN SCHNEIDERS