This seems like a recipe for many false positives and negatives.

One example: I live on the west coast but ran the Boston Marathon the year of the bombings. People closest to me knew I was there and we were able to get in touch eventually. Would this app know to alert my friends and family who didn't know I was in Boston?

Alternatively, we had a 6.something level earthquake near my home at a time I was traveling overseas. My grandparents eventually got in contact but they were needlessly worried since I was thousands of miles away from the event.