How does a looking glass / BGP tool know that someone is peered with someone else whenever that peer isn't announcing any prefixes from that ASN?

I've noticed some online tools (bgp.he.net in particular) can show who is providing transit or peering, and I'm wondering how is it that some of these tools can see who I'm peered with via an Internet Exchange when my ASN is looked up. I'm guessing it's happening with Route Reflectors/BGP at some point, but I was curious as to how an outside network can see who is peered point to point like that with no transit or exporting of the routes.