current activity automatically displays in profile for all to see when someones profile shows they are online.

it does not show up when you set your profile as invisible, and others are reading your profile...

that does not mean someone cant tell what you are doing. if you are posting while invisible your posts show up. for frequent "surfers" of the boards they know someone is invisible but here by how your posts show up (there is a timed stamp on all posts that keeps posts in order of when they were posted)