Browsing the Wiki, I found this.
After 30 minutes of acting as simple threat, these ships will join up with the threatfleet.
This imply there are "simple threat" and "threatfleet" and they are not the same.
I thought there were only one behavior for non-guarding, non-special-forces AI units: roam Human frontier and backstab when possible. I supposed awakened guards, CPA, preemption, Cross Planet Waves, etc were all assigning their ships to that unique threat thing.
Also, I barely remember, now that I'm thinking about it, there is another state where AI ships may patrol a small set of planets. The F3 key gives some interesting info (and often undecipherable to me) when the mouse hover over AI units. Does someone have accurate knowledge on that?
This might be of crucial importance because I'm afraid I wrote completely misleading info (if not plain mistakes) on the wiki (here