If you carefully search around the Ore Belt sector in the asteroid field about 70km north of the north gate, you will find an exceptionally large hollow asteroid. If you slowly and carefully fly through, your scanner will eventually find a hidden container. If you pick it up, you get 0 new cargo, but a random amount of credits. Ive tried it 4 times now (using salvage insurance and reloading the game) and Ive gotten amounts between 153,258 & 736,932 Cr.
In an older game, i searched the same spot but the container wasnt there, so Im pretty sure this only works before the