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If you have already opened a case for an unauthorised transaction then all i can suggest is to do an anti-virus scan on your computer as well.
I'm sorry to hear about the unrecognized transaction. If the transaction was made with a site that you have used in the past, is it possible that you established a pre-approved payment with that site? If there's a password on that site, it's possible that your password for that site was compromised. If that happened, you would want to change that password as soon as possible and consider canceling the pre-approval through PayPal.
You can check for pre-approved payments by following these steps. If there's a subscription listed there, you can use the listed options to cancel it.
I hope this helps!