I have a remote site that was converted to a different type of connection, but we're peering with the same ISP.
The following settings (summarized) are on the interface of the remote router:
network 10.5.5.0 mask 255.255.255.0
network 172.21.9.0 mask 255.255.255.0
neighbor 172.30.9.2 remote-as xxxx
I can ping 10.5.5.x, and I can ping 172.30.9.1. The network statement for 172.30.9.0 isn't in the BGP process, so I *assume* that it's because I'm peering on that same network is why I can see it? The ISP would be peering with 172.30.9.1, so I'm assuming that's why I can see it.