The plain clothes officer team were recovering a stolen car yesterday (Monday) in South Folds Road, in Oakley Hay industrial estate, Corby, when they realised they could smell cannabis.
They made some enquiries with neighbouring units and found an open fire door at the back of a unit and discovered the large, industrial-scale cannabis factory.
The two units had been leased in July, so police estimate the criminals had one harvest from the plants before the site was discovered yesterday.
There was a large number of plants across the two units and split across two levels.
Police said each plant would yield £3,000 worth of crop each harvest and would harvest four times a year, making each plant worth £12,000 a year.
The offenders had altered their mains electricity access to supply the operation.
Across the two units, the offenders had constructed living areas as well as crop growing areas.
There was a bedroom and living area with painted walls. There was even a shower upstairs.
Police did not find anyone on the premises.
A policeman on the scene said they would be working with the building company to find out who the leaseholder is to further their enquiries.
Staff in neighbouring units said they had been able to smell cannabis for the last few weeks.
One man said: "Sometime last week, a guy from across the road came over and said 'have you been smoking that wacky baccy?'
"I couldn't really smell it until a couple of mornings ago."
A man from another unit on the road said: "The last two weeks, it's been reeking. We thought someone was smoking it.
"You can still smell it now.
"We've not seen a single person nowhere near [that unit]."
Police are seizing the contents of the warehouse and will be destroying the cannabis and items used to grow it.
There have been several cannabis factories discovered in Corby this year, including one in Weldon Industrial Estate in February where around 1000 plants were seized.
Another one was found in March in Edison Courtyard, off Brunel Road where around 1,500 cannabis plants were discovered.
Although not in Corby, another big cannabis factory was found by Northamptonshire Police in Kettering's former Gala Bingo in June.
It was the county's largest ever cannabis factory with plants worth £2.8m and Northamptonshire's Chief Constable, Nick Adderley, said: "Drug dealing is one of our priorities and we’re focusing attention on the suppliers of cannabis.
“We’ve shone a light on it and we’re going to deal with it.”