The "we haven't seen them in a while, so we must assume the worst" almost sounds a bit too cavalier or insensitive to me. If someone does set this up and really does die, the email could be hitting grieving relatives / friends, etc. In addition, an email sent by that person is likely to be very disorienting.

I think I'd rather receive a more direct message about what exactly is happening. "So and so set up a Dead Man's Switch to notify you if they do not respond to us for a certain period of time. That period of time has now elapsed. If you need something..."