I had a very simler experience when my mother passed away 6 years ago. Me and my bother were sitting next to her bed in the hospital when she looked right at us and clearly said" I don't suppose you talk much to your father" [her husband of 54 years, gone 3 at that time] then kind of looked away and shook her head as if thinking of course THEY can't. Then the next day,she said when did mom get here? obviously referring to her mother[gone 30 years]which struck me because she never referred to her as "mom" to me and my bother,only "Gramma". Mom was for when she was talking to her bothers and sister. Seeing our startled looks, she replied very sharply"do you think when people die they just go away? of course they are still around"
My bother,being the analytical chemistry professor he is, dismissed it as a result of the pain meds. I was not so sure. Hearing that it is a apparently common experience makes me even less so.