I have a question about aim? ?

Today I went on one of my old sns and I noticed on the buddy list that oen of my friends sns were signed on that he never uses and had not used it for liek 2 years. So i talked to the person and they replied and they said that they had the sn the whole time when it was someone who we had not idea. It was no one that me or my friend knew. Do you have any idea what that could have been.