At my FIL' s funeral, my MIL being in an already advanced state of Alzheimer's, thought she was at a party. She did keep asking for ... and was told "He's here somewhere". We kept that up for a while afterwards too "He's moving the car" always seemed to satisfy her. Then she stopped asking for him.
For years, before she stopped talking coherently, she thought her son (my husband) was her brother, and I would find it so touching the way he would sit with her and "reminisce" about their school days, or their childhood. He basically went wherever she was in terms of conversation. It would make her so happy.
Sadly she can no longer communicate at all.
It is the saddest illness.