Can we truly trust the accuracy of such a sign? A broken clock is right twice per day. That sign is only right when you and the sign are in proximity to each other. If you leave, the sign probably does not change to say "You are not here." (Video surveillance when no one is in the sign's presence may be needed to reduce the chances pf any Schrodinger-esque observation issues further complicating the problem.)
A much more accurate version would state something like:
If you are reading this sign, then you are probably fairly close to it.