An unattended death is a death of a person where the person died alone.
For example if someone lives alone in their home and they die of natural causes and the person is not found for a few days to a week or so then that would be an unattended death.
Most unattended deaths happen to people who are elderly and live alone and have little to no visitors coming by their home to check on them.
Eventually they die and then they are not found for weeks and sometimes months or unless a neighbor who normally sees the person out hasn't seen the person in longer than a week or less then they call the police to do a welfare check.
Then the police find the person dead and then writes on the report unattended death.