UFO Spawn rate

I have only seen them during my first visits to neighbours (when all three actions are still available to do). I am under the impression that is when they are most likely to spawn. I've only heard of one spawning on a second visit to a neighbours town once. I'll try to find the thread and post a link to it for you.

EDIT: Here it is: http://forum.ea.com/eaforum/posts/list/10220526.page