One thing to bear in mind is that the when the Autonomo enters sleep mode it shuts down the USB peripheral on the MCU. This will close the session in most, if not all, terminal programs (PuTTY, Arduino Serial Monitor etc). After the Autonomo exits from sleep, a new session will have to be opened with the device (closing and re-opening the Serial Monitor will do this).
It is also possible to detect if the USB cable is attached and only enter sleep if it isn't. Additionally, there are commands to detect if the cable was reconnected while in sleep mode. This effectively will wait, after waking, until the connection with the host is re-established.
The detach command disconnects the device from the host. If this is not used the host will think the device is still connected and will likely give some error do to a lack of response. (I often see messages stating device is not recognised).