I would have to say that 198.162.100.1 is probably a firewall or a router doing NAT. Furthermore, whatever the device is, it is not responding to ping (which would make me more likely to believe it is a firewall), due to the "request timed out" message.

The .1 address is also indicative of a gateway device usually

So what is happening, is a user from within the remote network is trying to connect to your network. You see the .100.1 address because this is the address that all internal IP's are being NAT'ed to. Since the source ip is being translated, you will more than likely not be able to know the actual IP it is coming from. If it really becomes a concern, I would notify the admin of the network, and he/she can view the translation tables on the gateway to see where the traffic originates from. If all else fails, simply block that IP at your FW.