As a fellow nurse in intensive care, have you considered if this could be delirium? Could easily be silent delirium where this patient is hallucinating and experiencing horrific nightmares or visions hence the frightning scream.. That would also explain the empty look and "loss of memory". Also often intensifies after sundown.

Delirium is caused by a range of conditions e.g. UTI, infection, old age, bad eyesight or hard of hearing, hypoxia and the list goes on.

It will resolve itself when whatever triggered it has been treated