Even if I user enables "hide my online status in this session" others can check his presence by checking user list or profil and observe last visit time. By refreshing the page, can see how the time of presence changes / updates. The fact is that this behavior reveals the status of the user, even if "Hide online status" is enabled.
After logging in with the marked "hide online status in this session" please display your own profile. You can see the current time as the last visit time. This time is updated if we do something on the forum, for example, write a post or enter the UCP. Everyone can observe it in another user's profile. This is not good in my opinion. This time should show the previous visit, not the current activities.
If user now mark "hide my online status" in the UCP and display his own profile again, he and anyone will see "-" instead of the last visit time. It's OK.
In my opinion, a hidden session and its activities should not be shown in the profile as the time of the last visit. Alternatively, this could only be seen by administrators.