Should I see noise on an outlet ground?
The result is not so surprising if you consider what you are actually measuring. If you connect the probe tip to mains earth and the scope itself is earthed via mains plug, then you are just basically measuring the voltage over a loop of wire, basically a coil with a single turn. Changing magnetic fields through the loop induce voltage on the wire which might be what you are measuring. Also between the two measurement points on the earth wire (scope mains plug and probe tip) there can be earth leakage currents of other devices flowing in the wire and these get converted to voltage by the earth wire resistance. The metal box can also act as an antenna so it might receive radiated noise. Or it could be capacitively coupled noise, just like what you see when touching the scope probe tip with a finger.