Don't know if I am in the right bit to ask this question but... my partner use to have an email account which he has not been on for months (so am aware) now yesterday a messenger plus log chat appeared on our computer saved in our documents... now he was very distressed that this had suddenly appeared and he swore blind he hadn't held that conversation or been on that email account for a year at least... I believed him... however how can it just appear? Did he really have that conversation or is it some kind of virus... really confused.

did he really have that conversation or is it some kind of virus...

Update your virus definitions and run a full virus scan. You do have antivirus software right?