Does a service, for example, SSH, generate traffic when only listening (e.g. does it advertise itself in some way) when there is no connection established?
I ask as on Android devices for example, we can monitor incoming and outgoing traffic, great - we can block specific requests, IPs, app permissions etc. but even when doing this, could there be a service listening in the background (hidden in the firmware) such that the device can be accessed? Then all the monitoring/blocking would not be an effective defense, if the service generates no traffic (or even very little such that it is not noticed).