Martian Source from unconfigured interface

There is a cable plugged in to the second network card, but that interface is not configured. I’m getting a lot of “martian source” alerts because of this.
If the interface eth1 isn’t started because it isn’t configured, why am I getting these messages?

Martians happen because packets arrive on interfaces upon which they are not expected. Do you have both interfaces attached to the same switch? Why would you want to do this? If you want multiple networks on one wire (which is OK), configure secondary addresses on the single adapter you use to connect to the wire.

I know what martian packets are, but the question is “if eth1 is not configured, and ifconfig says it’s not up, why would anything be reported?”
Why is anything even monitoring the port at all?

The hardware was installed and connected before the OS was installed. The messages were noticed after the OS was installed but before the two cards were bonded.