Mr. Captain Bob:
If I understand you, you're saying that a duplex receptacle in a metal (?) junction box was providing 4.4 volts AC even though there were no wires connected to it?
I wouldn't have a clue either.
This is just a SWAG (Scientific Wild A$$ed Guess), but maybe put your tester back in that outlet, and have someone phone your grandson's house. See if the voltage you read shoots up to 90 volts when the phone rings.
I don't know what voltage telephone wiring operates on when the lines are not in use, but I know that the ring signal is a full 90 volt signal, and can give you a pretty good jolt from a tiny wire like that.
I'm thinking maybe a nail or drywall screw is connecting a telephone wire to the metal jacket of the armoured cable going to that junction box.