Officers in the United Kingdom say an industrial warehouse had all the “classic cannabis factory signs,” but when they raided the place, they found computer equipment.

Investigators with West Midlands police observed a lot of wiring and ventilation being installed in an industrial unit, in addition to people frequently visiting the unit at odd hours, and when a police drone flew overhead, it detected a considerable heat source.

After getting a warrant to investigate an illegal cannabis operation, officers were surprised to find a lack of green in the unit.

Instead, officers say, there were about 100 computer units being used for Bitcoin mining.

“It’s certainly not what we were expecting! It had all the hallmarks of a cannabis cultivation set-up,” said Sandwell Police Sergeant Jennifer Griffin.

They say the warehouse was stealing thousands of dollars worth of electricity off the main supply grid. Sandwell is in the Birmingham, England, area.

“My understanding is that mining for cryptocurrency is not itself illegal but clearly abstracting electricity from the mains supply to power it is,” Griffin added.

No one was at the unit when the raid happened, and no arrests have been made at this time, according to police.

Bitcoin mining is the process by which spent or purchased Bitcoins are entered into circulation and given value, basically a transaction record of the exchange. The process is necessary for the cryptocurrency to exist. It sometimes comes under fire because of the massive amount of energy required to complete transactions on the blockchain.