I thought that Hosts aren't a physical thing at all and they are purely matrix constructs. However, physical devices can slave to a Host in order to share data and have IC protection. So you wouldn't be detecting the host, but terminals/commlinks/sensors connected to the host. To do that, you do a Snoop action and use Electronic Warfare + Intuition vs Logic + Firewall. Now, to go in the other direction: If you're on a host and you want to find a physical device (e.g. slaved camera, terminal, commlink), it would require a Trace Icon action using Computer + Intuition vs. Willpower + Sleaze.