Maybe someone just sustained an injury and it was never cleaned. If this is anything like the SEEDs ship we were born on...

[ he remembers quietly. those long, quiet days spent with just rem, as... ]

--Maybe there's a hidden place we haven't found yet. A room that has the crew in cryostasis. That would make the most sense, right. Humans can't last long out in space, so they'd have to do something to keep from aging too much after long space travel...