Sounds like it’s a matter of tracing your water pipes to see where it is picking up the current from.
Worst case it’s in the ground outside - but that’s unlikely if it is only the one faucet.
Next worse case is some drywall has to be taken down to finish tracing the piping and find the current source.
You can get a lighted scope with an application for a smart phone, if you have one, so you limit the damage by only putting in a small hole.
I’d check the electrical on the sump just to make sure, but I’m leaning against it if it isn’t affecting the other parts of the facility - but maybe the bathroom is closer and the path of least resistance.
Tracing to find the source is the only thing I can think of at this point. Might get a meter out to check the current level. 5V on a metal circuit with minimal resistance likely doesn’t have a high current so in my mind it’s likely a nicked wire. current is coming from somewhere.
We also found 8 volts at the copper pipe entering the water heater in the basement. No voltage on the copper pipes in the baseboard heaters upstairs. The house has one water service but two meters—one for the English basement apartment, and the other for the rest of the house. The heating system is a 4 zone baseboard water heating system. A zone for each floor.