Oh wow, that's a fun one. It sounds like the network card was in promiscuous mode.

The wikipedia page even has a bit about this.

https://en.wikipedia.org/wiki/Promiscuous_mode

>As promiscuous mode can be used in a malicious way to capture private data in transit on a network, computer security professionals might be interested in detecting network devices that are in promiscuous mode. In promiscuous mode, some software might send responses to frames even though they were addressed to another machine.