Easy solution....disconnect incoming services from the house, and meter in both directions...if voltage on the F1 side(old days-thats the utilities' side), if on the F2(Customers side) its in the house! Good Luck!!!!
Discount each line of the splitter and cable box or router. Check continuity for a short. Could be a sloppy install with a staple(s) shorting the line. Test by using a meter or a line tester for coaxial cable.