You could get someone to check when you last logged on before you log in. You could then see if it coincided with the last time you remember logging in.
alternatively, you could change your password.
For security reasons, he
should change his password.
What does the strange pms contain? Maybe we can extract something essential or anything to that. If there is a link or any misleading information then you should send some message to the recipients to correct it and say that it is not you or the drunk you or whoever do that.
Can't you tell if it's you base on how the pm is constructed? If so, then its weird.
