Thank you all for your answers and discussion. Unfortunately it’s a “real problem”. There is a person who I trusted before but this person is now in suspicion for a bad deed. While changing my passwords (way too late I did that) I saw a log in to my personal account that was definitely not made by myself. It’s possible that that person had an auto login but I also had the hunch this person spied my personal mailbox (which is of great concern because I was in touch with official entities). Well I think the chance is quite low I forgot to logout somewhere and that that device has the same /56 prefix as that person. So I can just hope that was an auto login or that person did not found anything. Thank you all.