In the final scenes of TNG: "Transfigurations" (after meeting with Zalkonian ship), The Zalkonian Commander Sunad explains, that they have hunted down small ship, on which the alien ("John Doe") and three others escaped Zalkonian home planet, destroyed it and killed all abroad. The fact, that Enterprise crew has rescued John Doe was a real surprise to Sunad.
It doesn't make sense to me, what, in this situation, caused Zalkonian ship to return to the place, where they once destroyed refugees' ship? If they were so sure, that both ship and entire crew was eliminated, that shouldn't have any reasons to return. There's also no sign in the episode about anything, that would let them change their mind / become suspicious / detect the alien.