Could they have been gibbons?

About 3 years ago, while hiking up the Jhihben River gorge in Taitung, way up the river, far from the hotsprings in a very remote area, my friend saw a group of monkeys high up on the wall of the canyon. How he spotted them, I don’t know, but luckily we had binoculars, and what we saw, I swear, were not macaques. There was a nest of 3-4 what I can only say were white crested gibbons. Is it possible that someone had them as pets and let them go? The article linked below talks about how Taiwanese people used to buy gibbons in large numbers. I remember reading in one of the Chat Room textbooks that an orangutan attacked two hikers in Taichung. Could there be a breeding population of gibbons in Taitung? … taiwan.pdf

I’ve seen lots of gibbons here, but never any in the wild. It wouldn’t surprise me to know there have been a few escapes that have formed breeding pairs.