How can one be certain of the separation of semen from its place—which necessitates ghusl according to the Hanafis—in the case of waking up with the penis erect, yet unaware of the separation?

If semen moves from its place with desire but does not exit, it does not necessitate ghusl (definition) according to the majority of scholars. The movement of semen is the feeling of it stirring from the loins and being ready to exit upon feeling desire. If a sleeping person finds semen on their clothes or body, ghusl becomes obligatory for them, even if they do not recall a wet dream.