Proxibook is a proximity-based, contact-aware app that allows you to create reminders associated to your contacts and then be notified when you are near them.
It runs in the background and uses Bluetooth only. It does not require any wifi or cellular data, nor any hardware accessory in order to function properly. Simply make sure your contacts have the app too.
Moreover, it uses your phone number to identify your device over Bluetooth, and it saves your reminders on the device only and does not upload them to the Cloud, so you can be rest assured about your privacy.
Proxibook requires access to Contacts, Bluetooth Sharing, and Notifications.