Shoutbox

(CyBeRDuDe), Plus! uses the absolute minimum packet sniffing, and most if not all events are recorded via various APIs that Plus! has found inside Messenger. Anyway, even if the PC is acting as a router, Messenger will still only receive traffic sent directly to it, not to any other Messengers running elsewhere on the network.
The way the PC acts as a router is just like a real router works: it only forwards data to the PC itself if that data is meant for that PC.
Also, when using hubs and switches, PCs reject packets that are not meant for them.

Basically, Plus! is catching the events via API, using packet sniffing only where the API is broken (which is On*FriendlyNameChange in 7.5). So, these spurious entries in the log are genuinely sent to that individual Messenger, and will be of people on your Reverse List that you aren't otherwise aware are on there.