It's undoubtedly not worth your time and I hesitate to even mention it, but apparently on the night of the Boyle Party someone signed the guestbook as "Corvo Attano". It must be a perverse joke, surely. However, you asked to be notified of all such matters, and in truth we never located the Royal Protector's body, so I thought best to bring it to your attention. Sorry for troubling you with this matter. I know Lady Boyle was dear to you.
If Corvo both chooses the lethal elimination and signs the guestbook in Lady Boyle's Last Party, this note will appear on the desk in the Lord Regent's quarters in Return to the Tower.
An alternate note will appear if Corvo chooses the nonlethal elimination and signs the guestbook.