If I witnessed it correctly, then a message will appear if a boss spawns in a zone regardless of the channel you are in.
Either way this leads to cheap and effective ability for guilds to claim worldbosses for themself.
On EU for example you see one cerberus announcement per day - in the middle of the night or early - which is a bit unenjoayble. Make them at least work for it by running every channel again and again until they grow tired of it.