An alert server is operative to notify users when they are in proximity within one another. The alert server may determine to alert a first user of the proximity of a second user when the second user is in an unusual location. The alert server may determine that the second user is in an unusual location when the second user is outside his or her routine geographic location. The alert server may determine whether the second user is outside his or her routine geographic location by assigning confidence values to geographic locations that the second user has previously visited. The alert sent to the first user may be in the form of a text message, e-mail, or other electronic communication. The first user may receive the alert on a mobile device, such as a cellphone, smartphone, netbook, or other mobile device.