Most apps that react to where you are do it the easy way: the phone reports its position to a server every few minutes, the server decides, the server sends a notification. That works, and it means someone else holds a record of everywhere you have been. Brain Buffer does the deciding on the phone, and sends the position only for the one kind of reminder that cannot work without it.
You save a place — home, the office, a particular shop — and the app keeps its coordinates and radius on the device. Every minute the background service compares your current position with that list locally. When you cross into a saved place, the phone itself raises the notification.
That check makes no request to us: your position is not sent for it. The place itself — its name, coordinates and radius — is stored in your account, so the same place works on your other devices, and when a reminder fires the phone tells us which reminder it was, so it appears in your history everywhere. What we do not receive is a stream of where you are between those moments.
A satellite fix is unreliable inside a building, so a place can also be recognised by the WiFi networks around it. Saving a place sends the list of networks around you once; what is kept is a hash of each identifier, never the identifier itself, and a place matches when at least two of them are visible. The matching afterwards is local.
A hash makes an identifier harder to recover, not impossible: it is a fingerprint of the network, not its name or its address.
Reminders about a type of place — any supermarket, any petrol station — cannot work this way, because the phone has no list of every supermarket in the world. For those, and only those, the app asks the server what is nearby — about every two minutes, and only while such a reminder exists and is not paused. Those coordinates are used for the lookup and are not kept; delete or pause the last one and the phone stops sending them. You can use the app without ever creating one.
You can set a reminder for someone in your family — the shopping, walking the dog, the parcel. It fires on their phone, from their own copy of their own places. Your family cannot see where you are: a family here shares reminders, not locations.