I think there is some confusion going on here between what the OP asked, and the reason for the asking (to pack the CPAP away in a secure place from small prying hands
the joys of small children in the house
).
The Dreamstation doesn't have a "power on/off" button, it has a round button on the front that accesses the various menus, and a flat button on the top that starts/stops therapy, and a small flat button next to that which switches on the Ramp mode.
The flat therapy button is as close as it gets to a "power down button on the Dreamstation.
To disconnect power the options are- unplug the lead from the unit.. or turn off the wall socket.
Even with the wall socket turned off and the plug pulled from the wall the transformer will hold residual current for a short time.
There is no difference in effect on the Dreamstation by disconnecting in any of the three method options.
The only variation is if you turn off the wall socket/remove the plug from the wall, then the machine will deplete what little residual current remains in the transformer.. but it is not necessary to do this when disconnecting the machine from power.
Even unplugging the power while the machine is running shouldn't cause a big issue, most modern electronic devices can survive an "instant blackout" scenario (though I wouldn't make a habit of it).