Floating voltage on metal shelf
This is perfectly normal and the reason why metal furniture is tricky. Your shelf is at earth potential while your Xbox PSU has no earth connection, which is why its GND is floating. This means both its live and neutral wire are connected to GND by a Y class capacitor in the filter circuit. As long the current is only a few 100µA, that's ok by design. It may be unexpected though.



This (roughly) is the schematic of a LVDC PSU with the common filter caps. The Y capacitors are pretty useless when you are having no earth connection on the mains side but PSUs may come with or without earthed plugs due to local regulations so you can assume the Y caps are there even when there is no earth. This causes the inner shield to float at half of mains voltage at a very high impedance. 

On the low voltage side, there may be also caps connected to the inner shielding which causes the "half on mains" voltage to be displaced on the DC lines.

For medical devices this is often not acceptable so there are regulations which force medical PSU suppliers to reduce the Y capacitance (even when there is earthing) and add a big choke coil instead for proper mains filtering.