I believe that you're only explicitly notified in the log if you have a ship or planet in the system that the warp point appears in, but if you've got that area of space mapped you'll see the line appear on the map regardless of your presence there. Since you're ancient race, you'll see all warp lines appear or disappear wherever it may happen. The same applies to planet creation and destruction, explaining how you could watch that Irha-Nrr Maker class ship chewing its way around the asteroid belt in Cetkirk excreting planets as it goes even before you sent that scoutship there.
