If you disable the setting to allow others to see that you are online (under profile ---> Account related settings) then your 'actions' will not be publicly displayed.
Does "guest" imply somebody who has turned this off? I initially thought guest referred to somebody who has not logged in, but is browsing the forum?
Is there any shortcut to find out what a particular user is reading? One way is to sort by name and then go to a particular user's name; but this is not useful if you want to continually track a user.
I would think that it would be possible to make a monitor program to conduct such surveilance, however, I'm strongly against that, and I would rather have this feature turned off.
On another note, if you did such monitoring, and someone claimed not to have read a thread, but the database of your analysis said otherwise, you might point to the evidence, however that's kind of a strange exercise to indulge yourself in.
Those wanting privacy knows already how to protect themselves.
