Also, is there no email confirmation required for withdrawals on Kraken? That's typically a very basic security requirement that all exchanges employ. The OP says their email account was not compromised.
How can there not be? Kraken's
guide to request a withdrawal of any coin points out that a withdrawal can only be completed by clicking the confirmation link in the email sent to you.
If my email would have been hacked they would have changed the password at first and they would have deleted the emails - but they we're all unread in the inbox!
Read emails can be marked as unread super easily.
I'm inclined to think that in some way a hacker got access to your computer well before your coins were stolen, all to wait for the right moment to strike. If that turns out to be the case, the hacker has had all the data you have to access your account/withdraw/make account changes, etc.
I'm very curious to see Kraken's explanation after doing some investigation. They should know what exactly happened by the detail.