Mail Left On Radiator in Vestibule

I live in an apartment building. I came home today to find some pieces of mail sitting on the rotator in the vestibule of the build. The vestibule is the area you walk into to get to the door you need key to open to get inside the building. I took the mail inside. Then I called the SO post office and told the person that answered the phone where I found some mail. The person said to me that it was put into the wrong mail box and the person that got it in their mail box put the mail on the rotator in the vestibule and I will talk with the carrier. I highly dought it. When the people my build get the wrong mail in their mail box, they always leave the mail on the table in front of the mail boxes. Anyone can just walk into our vestibule and take anything left in there. Then I saw one of my neighbors and gave her the piece that was for her. Then I told her about were I found some mail and she said to me every week we have a different mail person. My neighbor is a Collage student and is home during the day. I work, so I do not see the mail person. Has anyone ells had this happen to them? Why do we have a deferent mail person ever week? We used to have one mail person that delivered our mail and got to know the people in my building.