"Don't leave your keys on the table" is evidently a superstition in Sweden. According to the internet, the myth behind the superstition is an interesting and a cautionary one. This is the explanation given by one internet source:
"Back in the day, prostitutes would indicate to potential clients their working status by putting their keys on the table in a hotel or bar. Parents didn't want to tell their daughters 'Don't put your keys on the table, people might think you are a hooker.' So instead they told them 'Don't put your keys on the table, it's bad luck.
I don't know if this story is true. But I think it is really interesting folklore."