Bluey,
Have you got a link between PayPal and your bank account?
Is the sum involved quite small - like 2 or 3 dollars?
PayPal makes such a small payment into your bank account to verify it when you set up a link - if you haven't set it up, possibly someone else has taken your bank details and has attempted to create a link between their PayPal account and your bank account.
Of course, it could just be a bank error, or some other error where someone has mis-typed a sort code / account number.
But I would be on to your bank like a flash to report it as suspicious activity if I were you.
SD