My mother was told by the GP that she had Alzheimer’s, but TBH had forgotten by the time she got home maybe 15 minutes later.
The first sign in her case, was noticed by my sister when they were on holiday. She’d always been an avid reader, but sister noticed that she was starting the same book from the beginning, more than once.
At the time, having already been through it all with FiL, I didn’t want to accept it. My ‘penny’ only dropped with a vengeance some time later. My DM had always been very clued up about finances, and one day phoned her bank (First Direct) about something - and had forgotten, literally the instant she put the phone down, what they had said. ?