AtHome itself is a violation of privacy anyway. If you would read your TOS, they explicitly say that they can basically use your system for whatever they want.

So, them having devised a network of machines that run security scan "authorized-scan*.home.com" is not put passed them.

As for them to be able to detect a sniffer, possible, but not likely for them to be able to use it on such a widespread network.

They would have to have some serious access to your system to be able to detect your NIC in promisc, if I'm not mistaken.
This isn't something that you're able to send packets at and detect because it's a read only thing. Maybe I'm wrong..