“The WLAN light on my router constantly flickers, even when I don’t use it, is there a reason for this? I’m afraid I am being attacked by a hacker or something.”

There sure is a reason! Don’t worry, there is an access point which is regularly transmits the beacon frames it is similar to the network places and it will announce its existence to any of the potential clients. Hence, this will result in outgoing traffic from your router side this will also be there when you are not using it.