Unexpected nanny
Giving your mother or mother in law a spare key in case you get locked out, or there’s an emergency is generally a good idea. The unspoken rule is that it is only used in these cases and with your permission. Otherwise, things like this happen.
Imagine being in the kitchen getting dinner ready and being alerted by your nanny cam that someone else is in the house! Running upstairs to protect your child and finding out that your mother had just decided to pop in without telling you, is a recipe for a heart attack.