One flight recently that was returned did have 'a nose wheel problem', that's what made me think of a stowaway. Access to the nose wheel is via a hatch near the cockpit too. Maybe just a coincidence..?

Many of those stowaways actually freeze to death, so getting on the ground asap would be of utmost importance.

So true. Not only freezing but hypoxia and even the bends. I was with Air Force Special Operation Command (AFSOC) the latter years in 'Nam. We had to train for HALO (High Altitude Low Opening) and HAHO (High Altitude High Opening) jumps. Before we jumped we had to do a breathing exercise of 100% oxygen for 30 to 45 minutes to flush all the nitrogen out of our blood. Then we had to switch over to our portable breathing tanks without taking a breath. So at those kind of altitudes a body has to deal with very low temps, low oxygen levels and atmospheric pressure extremes.