You already know that the following information is stored on a magnetic key card from your hotel room:
- Your name;
- your home address;
- the number of your room with the dates of entry and exit;
- your credit card number with the date of its termination.
When you return the card to the hotel, it can be considered by all and sundry, and the information on it is stored until the next guest is rewritten.
Never return the card to the hotel. Take it as a souvenir (a penalty for this is not allowed) or carefully process it several times along the magnetic strip with a magnet captured from the house and check whether it is able to open the number after that.
This is police advice. Give it to your friends.