John McKinnon, who works for a national hire firm, was tasked with picking up the van in Coatbridge and taking it back to a depot in Glasgow.
But when he opened it up he said it was full of what appeared to be cannabis plants.
John, 33, from Rutherglen, said: "I got sent out to pick the van up. On returning to our office, I opened the door of the back of the van and it was full of cannabis plants and evidence.
"There was cannabis plants, lights, obviously they've raided somewhere.
"The boss phoned the police and they came out and interrogated us about why we were in the van.
"They recovered the van and have sent officers out to interview us.
"The van should not have been released to us.
"I picked it up from Coatbridge at 10.30 on Sunday morning.
"We've got to clear and valet the vans.
"After we called them the police came and recovered the van and took everything away.
"They said that whoever signed that van out is going to get their knuckles rapped. It's a very serious situation."
Police confirmed they were investigating how evidence, or "police productions," had been left in the vehicle.
A spokeswoman for Police Scotland: "We can confirm that we are investigating a report that a vehicle hired by us has been returned to the hire company allegedly with what is believed to be police productions inside.
"Enquiries are at an early stage to establish the circumstances and it would be inappropriate to comment further."