What was surprising, in truth, was how Thaids didn't find it after all those years. But the answer, despite the weird problem, was that it wasn't easy.
The barrier was invisible. It could be occasionally seen shimmering, but that was it. For the thaids to get past it, they had to be very lucky, being at the right place, no, the right spot.
Though Erik noticed something as they headed toward the breach, there weren't thaids around. It was weird considering that another mass migration was taking place. But even this had simple explanations.
Either something devoured them all, or New Alexandria's troops killed them all. Regardless, thaids died, and in both cases, the perpetrator left nothing behind.
A thaid would have eaten everything, humans simply used the corpses to make armors and weapons.