Or if their house caught fire.
Or if they get dementia, and just chuck it away!
My MIL had lots of money in her house, not as much as OP, but still a few thousand. She was obsessed with it and would hide it in rolls all over the house. Trouble was that she'd forget where she'd hidden it, so we'd get panic phone calls telling us she'd been burgled etc. Which meant we had to go down and search the house, and inevitably, we'd find rolls of cash in really weird places. Then for a while, she'd remember where it was, until a few weeks later, she'd move it and we'd get another phone call to say she'd been burgled.
As the dementia got worse, she forgot she had the cash, so wasn't obsessed with moving it around and constantly re-hiding it. We'd find random rolls of notes when we went down to help her do things, i.e. kitchen drawers, inside the microwave, etc., and she'd be surprised when we told her, as she forgot she had money and thought she was hard up (reading too much Daily Mail!).
Then one day, OH was doing some gardening for her and looked in her green bin before putting a load of cuttings in, and there were some rolls of cash in it! He did a quick count and it turned out to be about £20k! We asked her, and she said she remembered it, and that she threw it away because it was useless as it was "foreign" (which it wasn't - it was just new-style and all she could remember was how notes used to look years ago). Heaven knows whether she'd thrown more money away!
After that, DH found and took all the money from her and took it away. Now we "drip feed" it to her to pay for her window cleaning, groceries, newspapers etc., and drip feed it into our bank account when we pay her bigger bills (like repainting, new carpet, new sofa, etc) by our bank cards directly.
Years ago, we had an elderly neighbour who died at an old age. As is common, her house was a wreck, so it was sold to someone who did a full refurbishment. We got to know him and he said he'd found loads of cash hidden all over the house, again, tens of thousands which was a huge amount back then in the 70's! Most was stuffed in boxes up the chimneys apparently, but there was also boxes of notes under floorboards, stuck behind/under the kitchen sink, etc. I think basically there was so much he got the house for free in the end. There were no relatives, so he just kept quiet about it!