My FIL started telling us all about his 'housekeeper' (he was living with us) and how she was no better than she should be, he had actually found her in bed with a man, the trollop.
He could still be quite lucid then, so we let him go on (it was quite interesting TBH!) and eventually worked out where it had come from. He had recently been to stay with BiL and SIL for a few days, and we deduced that SIL would be the housekeeper, since she did all the cooking and cleaning, etc. , and since he was in the habit of wandering around at night, going into other bedrooms and peering at the sleepers to see who they were, he would have found housekeeper SIL in bed with her object of lust, BiL.
Either that, or by the same criteria I was the trollop - by then he had come to think that our house was his own. I would sometimes find him going through drawers and chucking out things he could not see any use for. 'What's this darn thing doing in here?' Had to check bins on a regular basis - only had to leave him for half an hour and he would have cleared out all sorts of 'rubbish'.