Had Loki realized he had gone undetected, he could have perhaps walked - or teleported - away before he was noticed. As it was, it took a moment of non-reaction before the fact that he had come upon someone far too distracted to notice him, and by that point the man - familiar, at least slightly; perhaps he was another from the complex? - was looking up at him, attention no longer miles away from here.
>"Is something wrong?"
Loki tilted his head slightly, “Nothing of note, no,” he responded, “I was simply... passing through.” Something like that, at least. It was not completely a lie. He glanced off up the small paved trail that lead through the park as though considering carrying on down it, waiting a moment before he spoke again without looking back over.
“It seems as though something is bothering you, however.”
One did not become known as the god of lies and manipulation without learning to read people. It took being able to tell at a glance what someone’s mood was, to know where best to apply pressure to their emotions, what words it took to flatter and charm them - and barbs it took to tear them apart without them ever suspecting his involvement. Loki was a master at it, and at the moment it was nothing more than idle curiosity that drove him to pry - no ulterior motives, no deep sense of compassion making him wonder. Just simple, idle curiosity.