Does anyone know why they can't climb back out? The fourth picture shows them crawling on the walls and ceiling around the pipe. The third picture shows that the pipe isn't smooth inside, so they could conceivably climb it. Zooming in on the fourth picture doesn't seem to show a gap between the ceiling and the pipe, but that is the only reason I can think of as to why they couldn't climb back out.
Trappers/hunters who work in the wild wrap plastic around trees so otherwise adept tree climbing animals such as mice can't climb up the tree to eat their stored food (when bears aren't around in winter who could bypass the plastic). I'd imagine the machined metal used for the vents serve a similar purpose. Just not enough traction to climb that high up.