There are some applications that spy on users and monitor their activities on WhatsApp, for commercial purposes and send advertisements.
These apps monitor user activity and can tell when someone else was. Additionally it draw graphs of user activity, as they send all kinds of notifications when the user interacts with the phone.
Among these applications, those developed by parents to monitor children, but at the same time monitor everyone’s activity.
According to experts, these apps cross all red lines for user privacy, and at the same time, they use data that the user himself allows them to access.