12 fuse box

There probably isn't are you using a standard globe test light or a multimeter.

Standard glode test lights can give a false indication but if used right can also be used as a good fault diagnostic tool.

One of the issues with modern multimeters is they are very high impedance....they place no significant load on the circuit.

So they can tend to read voltage where there is only leakage and no solid connection.

That leakage can come from a number of sources.
One of them is a poor earth.

If one of the earths is poor voltage can go out on one circuit thru the common poor earth and back thru another circuit.

There may also be some sort of second supply, like a second battery or a back up battery is some sort of accessory.

Or there might be something dodgy like two wires crushed together.

as has been said try with a test lamp.

cheers

Looking up your listed fuseholder it has the option of an LED to indicate a blown fuse - this fuse needs a load on the other side for a reference to ground to light up but it will not pass sufficient current to juice up your device, if you disconnect the load at the terminal block it should go out.

cheers