Any sighting of the elusive Big Cat?

This is false. As someone who has spent thousands of hours hunting bosses, I can definitively say that the spawn notice only comes up once and only for the first person to enter the map after it has spawned. People use this knowledge to their advantage at places like Seruf, by ccing through all channels before killing them. This discourages passerbys or potential hunters. Also makes the hunt a little more painful, especially if you aren't paying attention to what channels you've visited.

I can confirm. You can clearly see in my above SS that I DID NOT receive the spawn message when I cc'ed into a channel already containing a Jaira.