If you get a message stating, “Your Apple ID was used to sign in to iCloud via a web browser.”
This is an alert from Apple to inform you that your account has been accessed. It includes the date and time that the account was accessed via the web.
What’s alarming about this message is that the time may likely be sometime late at night or early in the morning when you would not have even been awake.
The reason for the time difference is that the time is stated according to the timezone at Apple’s headquarters in Cupertino. You’ll see PDT next to the date and time in the alert.
As of early 2014, these alert messages may have included more information about the computer used to access the account, such as, “Your Apple ID (firstname.lastname@example.org) was used to sign in to iCloud on a MacBook Pro 15.”
By mid-2014 it seems that these messages no longer include the computer description.
If you don’t recognize the login as one of your own, it would be a good idea to change your password.