Key Derogatory

I lived in an apartment complex a couple years ago (in 2009 and 2010-moved out between the two and finally moved out in 2010). The manager of the apartment complex charged us some odd amount for cleaning fees in 2009 when we moved but NEVER sent us a bill. When we moved in in 2010 (we didn't have knowledge of the collection she sent for 2009) we paid those cleaning fees because that was part of being able to rent another apartment. In 2010 after we moved out again, we checked out credit and saw the two items on there. We were able to get her to remove the first collection payment, but she didn't removed the second one. We NEVER received any type of bill or phone call requesting payment for the cleaning fees in 2010. I disputed this with Equifax, but I'm not sure what else I need to do. It says it's Key Derogatroy right now, but how is that possible when we were never notified?