Was the cat spayed?
Had she been acting ill?
Do you live in any area with wildlife, like fox, or stray dogs?
Hello there yes she was sprayed ,and there is foxes about .
A cat this age will not typically run away. I have seen cats attacked and killed by wildlife, gotten locked in sheds, stolen, someone takes them home thinking they are a stray, animal control catches them, they get into poisons and hit by cars. If she had been acting off in the days prior, some cats wander off when they are near death as well. A 12 year old cat is not that old so hopefully that was not the case.
Thanks for your help.