TS, I can think of several possibilities, either one of the other human players or even an AI one, that he has a treaty or partnership with has shown him your part of the map, or one of the players that has a higher cloak then yours found your system and gave him the info without your knowing it, or Last, he is planning a major offensive into the system he is fighting over or planning to invade yours thinking it belongs to the same enemy he is fighting. I don't know if there is a cheat but I guess it could be possible, but you may want to check out the other possiblities first.
PS. Growltigga, I took your advice and indulged myself in a cool shower, after all the illustrious discussions that took place here, it was getting a bit warm and my wife kept asking me why I kept looking at her with a twinkle in my eye all the time,....

I took the bull by the horns and put the blame on ..... But it didn't work, after all these years she knows me to well,
ahh, women don't understand the intricacies of scientific debate, research, and experimentation

but like someone once said, "Tommorrow is another Day" heh, heh,
just some ideas mac