Alright, ready for some super sci-fi babble?
4-dimension quantum trackers.
A technology borrowed from an alien race, in order to track and warn about weapon above class 8 destruction potential (including nukes). These bizarre extraterrestrial computer systems are designed to travel the universe seeking class 8+ weapons, and locking onto them in the 4th dimension when they find them. This ensures ships without 4D access tech cannot detect or remove the tracker once it is attached. They tend to surround solar systems containing organic life or important territories and are able to search new ships that enter the solar system within hours of them entering. A receiver on nearby planets will be updated on all new nearby class 8+ weapons.