With MIL, it was loss of ability to do things she'd been doing for years. She stopped recording things on the video recorder, stopped using her cooker and using the microwave all the time instead, stopped reading books, couldn't write cheques anymore, etc.
Her actual memory was fine - she'd remember things as normal, you could have perfectly normal conversations with her, she carried on being self sufficient living in her own home, outward appearances were all fine.
OH visited a few times per week, nothing obvious at all, didn't even notice that she'd stopped reading, wasn't recording TV programmes anymore, etc. - why would you? It was only in conversations that OH would ask about, say, a TV programme that she said she'd missed - he'd ask her why she hadn't recorded it, but she'd just say she forgot. Same with meals, it was only when he looked in her fridge he noticed she had no "fresh" meats, salad, etc - it was all ready meals which she'd previously hated. Then one Christmas she wrote cheques to go in Christmas cards as normal, but the cheques were written incoherently. He started to ask more questions and take more notice, and that's when it became apparent that she simply couldn't do things anymore. He checked the video recorder and the programs on it were months old. Found lots of old "red" unpaid bills for utilities etc as they'd sent back incomplete/wrong cheques she'd posted to them, etc.
It was several more years until the memory loss, forgetfullness, etc kicked in, with her forgetting where she lived, reverting back to where she lived decades ago, forgetting she'd been married and had two children, thinking she still lived in her original home town with her brothers, etc etc.