What about "If you are reading this, there may be a serious problem. I usually log into my system everyday and since that hasn't happened, please call or try to find me to make sure I'm OK"
That way it's only slightly alarming to your loved ones.
that's different, but do you really need to worry the people you care about like this? bad news travels fast and if you do need to call for help there are ways slightly more person than a cronjob (though this sort of a cronjob message won't get you 5150'd, which generally makes whatever problems one has worse).
That way it's only slightly alarming to your loved ones.