Uninstalls are tracked by sending a silent notification to the available user push tokens once a day. Depending on the response from the push service, Leanplum determines if the token is unregistered due to app Uninstall and records that event in the user profile and analytics.
It is possible a user to have multiple devices - in that case the app can be uninstalled on one of the devices and still present on the other. In this case the profile will have an Uninstall event but the user would still be reachable on the other device.