I'm sure they do. Mom opened a unit in a neighboring county in 1989. She didn't open the door again until 2012. (Yeah, $35 a month times 23 years...) When she FINALLY let us in there to clean it out more than half of the boxes were either entirely empty, filled with other empty boxes, or mildewed from cat pee. My husband and I took one ton (a literal ton) to the landfill. The ONLY things of value were some old jewelry boxes filled with rings. The stones were fake, but the gold was real. He and I sold them to a pawn shop without telling her. Between that and the cash we were able to put down enough money to buy our house. Sometimes I feel bad for not telling her about the rings, but none were of sentimental value and we figured that if she hadn't missed them in 23 years...(all that money socked away and the amount she was paying in storage fees and yet I didn't have a vacation until I was 18 and the school had to pay for my glasses)
But yeah. I'm sure there are boxes full of garbage, too. Depending on the unit water and moisture can get in there if the stuff's been locked away for years.