Just because they’re not there on a walk round, doesn’t mean they can’t get through a hole in the fence and join the aircraft on taxy out. Not all airports are as secure as people think, particularly in the Third World.
I remember vaguely, a story where a stowaway had made it to Heathrow, and was found wondering around the apron one morning. The skipper of the flight who found out it was his aircraft the stowaway was on, was so appalled at this person’s desperation, that he went through the process to adopt him....maybe someone can add more to this, but it’s disappointing to see some of the joke comments made about someone taking extreme measures to escape a life somewhere in hope of a better one somewhere else.