it sounds like a problem with your power supply. Did you try another power supply i mean galvanic isolated (not switching PSU, but transformer based?). If you have PSU with on/off switch, turning off your power supply it usually does disconnect "hot", in this case 12-28V or mains 220V wire too, however ground remains still connected, so if there is some problem with parasitic current flowing from outlet to output of your power supply this should be it (it could be for example faulty capacitor in the circuit or something in insulation between HOT - Mains 220V and PSU output.
I can't imagine single reason why this could be routerboard related, when routerboard is not connected to any antennas yet.
In case it does appear when it is connected to some aerial system, that is another case, there is a posibility to induct parasitic current on the long wires and can be solved by another countermearures.