Do most sitters who stay over to watch pets typically leave the house key at the house when they finish their last visit?

My opinion is nothing is typical. Every agreement is personalized between the client’s/pets’ needs and the sitter agreement. Things usually work out smoothest if everything is discussed and agreed to before booking. It’s very possible that the sitter took with her and then forgot about it.

If this is a rover sitter, it may be a good idea to contact them. Ask if they can send a follow up communication.. Let them know she agreed to return the key yesterday, and today you still don’t have it back.