If a device is slaved to a host, can it be seen from outside the host?

Yes. (Word of Catalyst, read below for clarity.)

References If devices that are slaved to a host are not visible from outside the host, then a master host will never protect the device using its attributes since there is no way to access the device via a non-direct connection.
 * 1) Slaving a device to a host makes it part of the host's Wide Area Network (WAN).  The host's matrix attributes are used to defend against matrix actions instead of the device's rating. (Core p233, PANs and WANs)
 * 2) If you are in a host that has a WAN of slaved devices, you are considered directly connected to all devices in the WAN. (Core p233, PANs and WANs).  This is confirmed by A Lesson in Host Invasion (Data Trails p87)
 * 3) If you are directly connected to a device, it uses its own rating to defend against matrix actions, not its master's rating/attributes. (Core p233, PANs and WANs)

Rule #1 above would be moot as it would never apply.

Other relevant text

Core p224, Bank Example: BK knows that the device is part of the bank's WAN before he plugs into the universal data port. How would he know that without first examining the device via matrix perception?

Core p359 muddies the issue by saying that none of the devices can be accessed without gaining access to the host itself.