> The thing is multicast is not anycast, you will not receive multicast traffic unless you specifically ask to join a group.
Most likely a dumb cheap switch that doesn't snoop on IGMP (or a less dumb switch that wasn't configured to snoop on IGMP) was upstream of both the OP and the device in the elevator. So the frames were being flooded since the switch doesn't know any better, and then normally ignored by OP's network stack since they didn't join that multicast group, until something like tcpdump/wireshark enables promiscuous mode.
Well it's a bit of a mystery to me as well.
The thing is multicast is not anycast, you will not receive multicast traffic unless you specifically ask to join a group.
So, he would have to be actually listening somewhere in between the multicast router and the elevator?