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What your location history reveals about you

So far this series has explained how Fenzly protects your location. This post steps back and asks the question underneath all of it: why does location deserve that much protection in the first place? A position on a map seems harmless, “I have nothing to hide, I’m just at the supermarket.”

The trouble is that nobody collects a position. They collect a trail. And a trail tells a very different story.

A diary you didn’t know you were writing

Take a single, ordinary week of location traces and read it the way an analyst would.

The place your phone rests every night is your home address. The place it sits every weekday from nine to five is your employer. The route between them, bus stop or motorway, hints at how you live. So far, mundane.

Keep reading. A weekly Sunday-morning stop at the same building is a church; on Friday, a mosque; on Saturday, a synagogue. Repeated visits to an oncology ward, a dialysis unit, a fertility clinic or a psychiatrist’s office sketch your health. Evenings at another home address, again and again, name a relationship. A stop at a job centre, a pawn shop, an addiction support group, a political rally, a demonstration, each is a line in a diary you never meant to keep, written by your pocket.

None of that required reading a single message. Where you are quietly discloses who you are: religion, health, finances, relationships, politics, precisely the categories the law calls sensitive, all inferable from coordinates and timestamps.

“It’s anonymised” is mostly a comfort word

The usual reassurance is that the data is stripped of names. Research says that barely matters. A landmark study of fifteen months of mobility data for 1.5 million people found that four points in space and time are enough to uniquely identify 95% of individuals, even at the coarse resolution of phone antennas, not GPS.

The intuition is simple: your movement pattern is a fingerprint. Only one person sleeps at your address and works at your office. Knowing where “anonymous user 28471” spent Monday night and Tuesday morning is usually enough to give them back a name, yours.

This isn’t hypothetical

These aren’t thought experiments. Three cases reported by the press show what can happen to location data once it exists somewhere (sources at the end of this post).

In December 2021, the news site The Markup reported that Life360, a family-safety app, was selling the precise location data of tens of millions of users to about a dozen data brokers. In January 2022 the company announced that it would stop selling precise location data to data brokers.

In 2020, Motherboard reported that location data from Muslim Pro, a prayer app, was bought by a broker that also supplied US military contractors. The app then announced that it would stop sharing data with that broker.

And in 2021, a senior American priest resigned after his movements, extracted from supposedly anonymous app location data bought from a broker, were used to identify him and out his private life. One individual, picked out of an “anonymous” dataset, exactly as the research predicted.

The point is not to single out these apps. In each case the app worked fine and the users had consented to something. What these cases have in common is the existence of a database: once a readable archive of positions exists, it can be sold, leaked, subpoenaed or misused, whatever the intentions printed in a privacy policy, and whatever the company later decides.

The only database that can’t betray you

That’s the conclusion Fenzly is built on, and by now you can see the whole design follows from it. Positions travel sealed end-to-end and expire; the server is blind to your places, presence and circles; your history lives on your phone, for you and the people you chose, not in an archive we could monetise or be forced to open. There is no trail to sell because the trail is never assembled anywhere but in your hands.

The honest coda

One thing Fenzly cannot do: make your phone invisible. Your operating system and your mobile carrier see location signals below any app, that’s a different layer, governed by your phone’s settings and your operator’s obligations, and no app can change it. What an app can choose is whether it keeps a readable archive of your life on its own servers. We built ours so it can’t.

Next: a question worth asking of any location app, ours included: who’s paying for the servers? Our answer is you, openly, and that’s precisely the point.

Sources