Do Xbox 360 Slim power supplies output 5V until connected to Xbox?
The Power Supply does provide 5 volts to the xbox through what is known as +5VSB (VSB means Volts StandBy) this in turn is then shorted to POWER_ENABLE to turn on the power supply and activate the three 12 volt rails. This short only happens when you press the power button on the Xbox and it turns on. The short is then removed when the console no longer requires power.

The +5VSB is also used to power and charge anything connected to the USB ports on the console when it isn't on.