John McKinnon, who works for a national hire firm, went to pick up the van in Coatbridge and took it back to his company's depot in Glasgow.
But when he opened it up at the depot, he said, it was full of what appeared to be cannabis plants.
Mr McKinnon, 33, from Rutherglen in Glasgow, said: " 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 were cannabis plants, lights. Obviously they've raided somewhere.
"The boss phoned them up 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."
Police confirmed they were investigating how evidence, - which they refer to as police productions - had been left in the vehicle.
Mr McKinnon said: "The van should not have been released to us.
"I picked it up from Coatbridge at 10.30am on Sunday morning. We've got to clear the vans and valet them.
"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."
A spokeswoman for Police Scotland: "We can confirm that we are investigating a report that a vehicle hired by Police Scotland 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."