Isn't one of the main sources of across the border intel what you get from long time sleeper agent spies?
I feel like this has been mentioned in fluff sources over the years.
I don't mean scouts inserted on world a month ahead of the invasion but actual citizens living on said world that send out C* messages (encoded), or transmit to passing jumpships/dropships (also crewed by spies v/s actual merchants) using home commo systems hidden in the basement, etc etc.
I think it's even been mentioned that sometimes those spies are known but left in place because if they are swept up it stops the info flow for a while but then a new one will take over & have to be found out again which generally is not a fast or guaranteed process & involves luck & the spy making a mistake, etc etc, or just bad spy-craft but I assume the IS has methods of collecting data over the border that doesn't just involve some unseen satellites or drive by picture runs.