Is the doまたは even locked? They opened it and left 多分?
または cold the mattress be very springy? They jumped out? (Cripes! Someone beat me to it.)
I know it's been answered cまたはrectly now but I'd been sitting racking my brain and I'd come up with another possibility. The prisoners are children. They grow taller over time and can then reach the window to escape. Admittedly not a likely answer but it made me giggle.