Cops in the West Midlands of England thought they were about to bust a cannabis-growing operation. It had the telltale signs: mysterious visitors at different times of the day, visible wiring and ventilation ducts, and above all a significant heat signature, picked up by a police drone.

But when they raided the unit in a Sandwell industrial estate, the police found not a pot farm, but a Bitcoin mining operation.

That's not illegal in the U.K., unlike unlicensed cannabis cultivation—and unlike stealing power from the mains supply, which the Bitcoin miners in this case were doing.

"It’s certainly not what we were expecting!" said Sandwell Police Sergeant Jennifer Griffin in a statement. "It had all the hallmarks of a cannabis cultivation set-up and I believe it’s only the second such crypto mine we’ve encountered in the West Midlands."

Indeed, this is not the first time law enforcement has become confused between Bitcoin-mining operations and cannabis farms—reports of such mix-ups go back a decade. It's an understandable mistake to make, given the facts that cannabis needs artificial heating in colder climes and the high-performance computers that "mine" cryptocurrency, by racing rivals to complete mathematical problems, generate a lot of heat.

This heat can, if harnessed, prove useful. People have used it to warm homes, heat bathwater and grow "cryptotomatoes"—and, yes, to grow cannabis. A few years ago, an Italian medical cannabis grower called Evolution BNK even filed a patent for the heat transfer system it had developed to build combined cryptocurrency-mining and marijuana-cultivation operations, which are solar powered for good measure.

Griffin said the West Midlands Police hoped to permanently seize the Sandwell crypto-mining equipment under the U.K.'s Proceeds of Crime Act, due to the power theft.

It's not clear how profitable the mining operation could have been without stealing electricity, given the U.K.'s relatively high cost of power when compared with countries such as China—where the majority of Bitcoin mining takes place—and Iran, which has just banned cryptocurrency mining for four months due to its apparent role in causing major urban blackouts.

The profitability would certainly have taken a knock in recent months, given Bitcoin's recent loss of value. And, as some observers noted on social media after perusing the police's photos of the operation, the computers being used were inefficient by current standards.

Cryptocurrency mining's power-hungry nature has generated many headlines recently, after the environmental impact led Tesla CEO Elon Musk to reconsider his company's acceptance of Bitcoin as payment for its cars—partly causing the abovementioned drop in Bitcoin's value.