Please do not worry – you are not being hacked!
Since version 1.10.9 we added a “DHCP server finder” inside the FingBox. The purpose of this component is to discover every active DHCP server inside the network by sending fake request with the special MAC address “DE:AD:C0:DE:CA:FE”, which might be recorded as new device by the router. Knowing the list of active DHCP servers helps to troubleshoot some specific issues. This is generally considered standard practice for performing this sort of test.