[Suicide Junkie]

Cloaked ships could have passed by your main defenses. I notice that you did not include mines in your list of defenders.

If you don't have sensors, they could have been hanging around in your system for years.

If you do have sensors, they merely bypassed your planet while cloaked, then proceeded to attack your other planet.

If they had no vision in the system, they could have given an order to warp, then return. They would enter and leave, and you would not see them. Next turn they could have given an order to attack your other planet.