Constrictor wasn't invisible on the scanner. It was a tough nut to crack, and you had to chase it across several systems. Got a Naval Energy Unit for your trouble.
With C64 Elite the player was awarded the NEU after successfully delivering the Thargoid Plans to Birera in galaxy three. It was a long voyage from Ceerdi to Birera and I was jumped many times by Thargoids. But I knew how to dust the bugs real good.
The Constrictor mission played differently in Elite than it does with Oolite. The Constrictor thief is still found in the same system, but you had to go hunting for him. In Oolite, the player encounters him as they exit from witchspace. The Elite Constrictor is a tougher fight than it's Oolite counterpart. I believe the reward was in credits.
I encountered a few strange things playing Elite. Once I encountered a fleet of ships that were not hostile. I sat engrossed watching them fly by in single file. To this day I do not know what they were but back then I imagined it was a navy group out hunting Thargoids. Thargoid hunting was a passion of mine as well. My favorite planet for such an enterprise was Poor Agricultural, Corporate State. Pirates were not as numerous so they did not interfere. I would jump away from the planet and sun till they only showed as specks in the heavens. That tactic improved game speed or frame rate. The Thargoids inevitably showed up and I would tease them with the laser so they released all their thargons. Then I destroyed the mother ship and scooped the thargons which fetched a good price.