your sim card is basically your account, it holds the information about your phone number, your balance amount for phones that are "pay by the minute", and account information. you can buy a replacement phone and take your sim card out of the old phone and pop it in the new one - presto, new phone!
you can also swap the card between phones whenever you like.
most people tho, are not regularly switching back and forth between phones. but you can do it. one reason i can think of off the top of my head would be battery life. if you are constantly finding that your phone runs out of power you might carry another phone just in case...
so it is not completely unbelievable that casey would be carrying two phones and somehow lost the one that did not have the sim card in it. but it is odd, and something worth checking into for sure.