It happens a lot, mate. Chances are, they'll attempt to redeliver it. The notice should have been left in your buyer's mailbox, so there's a great chance he/she saw it and perhaps went to claim it at the PO. I wouldn't worry if I were you.
If indeed it does come back to you, I don't know what to tell you. I believe that the buyer has a certain obligation to accept the package once it arrives (as silly as it sounds to actually say this). If they're not around to accept the package, what are you supposed to do?
BTW, they're also not going to just leave a package on their doorstep (or at least they SHOULDN'T) if you used delivery confirmation. That's why they left a notice, not because it needed to be signed for.
Hope this helps.