I know some credit cards also have a 13.56 chip in them.
When you bring a card near it, an led lights showing it senses a card.
When I tried to see if it would light the led, it didn't
Again, I wanted to see if my entry system could be activated by a random RFID card,
No most credit cards do not have a contactless chip in them.
No the MIFARE cards have unique IDs on every one. Your system has to look at the number the reader returns not just the fact that the reader sees a MIFARE card.
Actually many do. On closer inspection I have 2. They can be identified by a WiFi-like logo on the card. Mastercards call it paypass or blink. If the cards have a public ID, why wouldn't the card be detected?