Alicia's "unconscious" body was too close to the door to allow the killer to exit.
Millie was right about Jason's state of mind. He had been ready to dump Alicia and return to his old love. But Alicia wasn't ready to let go.
While the others changed clothes, Alicia grabbed the ghost costume and the mask, plus the ax from the stairwell. She attacked Jason, but he fought back, cutting and bruising her in the process. The only way that Alicia could explain away the injuries was to pretend to be a second victim. She discarded the ax and costume, then pretended to be unconscious in the dungeon room.
Her one mistake? She lay down too close to the room's only door. If a real attacker had been in there, he would have had to move her body in order to squeeze out the door.