Not really indeed. You're right that false positive are possible with such a system, but false negatives are not. That means that you're leaking information abo

Not really indeed. You're right that false positive are possible with such a system, but false negatives are not. That means that you're leaking information about when a user didn't use a service, as well as partial information about when the did (which you could combine with other data to tell you something meaningful).