I often post stuff I don't want anyone else to know I've posted [which is the whole point of MN after all!] and my kids sometimes use my iPad, iPhone etc to look things up, using the same browser. I log out of Mumsnet, but the recently accessed topics are still highlighted which provides others with a clue as to what I may have beem posting on.
Once I have logged out, can you ensure that all topics show up as default colour, not highlighted please?