Oscilloscope shows a sinusoid V(RMS)=1.67V and V(MAX)=2.48V. I have a GW INSTEK GPS-3303 power supply and a RIGOL DS1052E oscilloscope. The supply is in series tracking mode, with the probe directly connected to the negative CH1 and ground of the probe connected to ground of the supply. The "output" button of the supply is in the off position. Master V=6.9V and I=1.06A.
At 50 Hz, you're picking up mains hum.
The voltage you're picking up is quite high though. This is often the case if your oscilloscope ground lead is not properly grounded (ie to mains ground). Since your supply has isolated outputs, you aren't connected to mains ground. When the power supply is off it's just acting as a big antenna for that 50Hz hum.
Still, 2.48V p-p is quite a lot, are you sure your probes not in x10 mode?
Why does my power supply show a signal on the oscilloscope when off?
If you disconnected the probe tip of the scope and left it unconnected to anything, what signal do you see? My guess is that you'll see a significant ac waveform and this is not unexpected.
You say your power supply is "off" but there is a good possibility that the output stage is disconected and your scope probe tip is just floating and picking up voltage from the air.
If you connected a light load (such as 10kohms) across your power supply output, does the signal disappear? My guess is yes.