Main breaker not turning off oven

Something very strange is going on at my mother’s house. The oven, which is not built in but just a pigtail plug has been malfunctioning. She has ordered a new oven and is ready to put it in.

When trying to cut power to the outlet, there are no breakers that work. She’s even had my brother go under the house to see if there’s another box somewhere and there is not.

They have cut off the main breaker to the house and everything is off but the oven remains on.