I think the Av-Matoran got out that way, also it's directly stated in "The Final Battle" that the Toa were trying to get out that way, too. As to how they knew about it - Vican told them.
In the book it seems like they simply teleported through or went intangible for a moment or something such:
The Final Battle pp 116-117:
They (the Toa) were heading for the portal in the western wall through which the Matoran of Light had escaped. (...) a wave of energy rocked them. Suddenly, the Toa were heading for a solid wall. (...)The Axalara, Jetrax, and Rockoh shimmered out of existence, along with the Toa, just long enough to pass unharmed through the walls.