Its nearly always a genuine transaction you have forgotten about.
Don't forget you can often pay sellers via debit and credit cards as guest buyers via paypal without logging in so it does not show in your transaction history.
I would double check there first though, go to activity and put in dates for the period in question and see if you can find the transaction, don't forget if you had funds in your paypal account balance they would have been used first so the amount may differ to what is on the transaction amount.
Ask your bank for more details of the transaction and you may recognise it.
If it is genuinely unauthorised then you can file a dispute via the resolve a problem link in your paypal account summary page.
Paypal will investigate it and the if they find in your favour you should be reimbursed, the time it takes depends on paypals investigation.
Advice is voluntary.
Kudos / Solution appreciated.
