As jaymor has said, you can get key fobs which let you find the key. As long as you can also find the locator control of course, which your mother would probably 'hide', possibly in the bin! So you would need to put the control somewhere she couldn't find it. And you would also need to depend on your mother not removing the key fob ("what's this? don't recognise it, so I'll remove it").

I never entirely solved the problem of my mother's lost keys. They were always found eventually (under her pillow was a good place to look). So in the meantime, the care agency kept a set of keys at the office, a neighbour had a set, and we also installed a keysafe. So the quick easy option would be a keysafe to keep a spare set, which will give you time to find the originals.

She had a Yale lock so she could always get out without the keys (which of course then meant she left the house without the keys and lock herself out ... hence the multiple spare sets available).