What happens if there is another bomb, after someone has reported as "safe"? Are the "safe" statuses updated somehow, since the state of that person is now unknown?
It's not happened, so I don't know if they already have it in- but it would be incredibly easy to look at the people who were reported as safe and specifically message them to see if they are or not, with a reset and a secondary check.
After all, you've got a perfect list of those who already responded.
100 falsehoods programmers believe about terrorist attacks.