Hi nick101,
Richiec1 is referring to iMessages as I have the same issue. Similar to him I have a work computer that I many need to return to my employer and I don't want anyone to be able to see that I have/had an account in iMessages and risk them re-activating it. Even after I signed out as you suggested. The email address that is my Apple ID still shows up along with Bonjour. Both show an inactive.
I also went to mail as you suggested and didn't know that there was an account active there as well, so thanks for your tip as I deleted that account completely.
Thanks so much for the help.
Elizabeth