When I program a recording to start directly after disconnecting the device, what is meant by "disconnecting"? In other words, when does my recording start exactly? Is it when I close the OM-GUI window, or when I safely eject the device (using the windows command), or when I manually unplug the Axivity sensor from the computer?

When you actually unplug the device from the USB cable the device will reset and start logging as per configuration settings.