When you have Nearby Emergency SMS Alerts enabled and/or Custom Alert Zones set up, you will receive text messages from either 1 (650) 661-0320 or 1 (650) 266-8668 in the event of nearby emergencies. You may also receive a text from 1 (650) 502-6403, but this phone number is very rarely used.
We suggest saving these phone numbers as contacts in your phone so you recognize them more easily (like in the image below).