Most banks have fraud alerts set in place. That is, when there is unusual activity in your account, many banks will contact you to make sure that it is you, the card holder, that initiated the activity.
These alerts do not only appear with respect to charges. I recently linked Rob’s JetBlue TrueBlue frequent flier account to my American Express Membership Rewards account, (because I can) and got the following email form Amex immediately after the transaction.
“Your Membership Reward account was recently linked with the Frequent Flyer/Frequent Guest accounts listed below. We would like to verify that you or an additional Card Member on your account completed this action.
JetBlue
If you or an additional Card Member on your account did not link these accounts, please call us at your earliest convenience using the number on the back of your card. Our hours of operation are Monday through Friday from 8:00am to 8:00pm Eastern Standard Time.
If you or an additional Card Member on your account linked these accounts, no further action is needed.
Thank you for your Card Membership.
Sincerely,
American Express Account Protection Services”