This is not of my own knowledge, but it is clear enough I would personally go with it. I acquired a list of aliases from a knowledgeable source, and this is included in that list:
alias unlock='sudo rm /var/lib/pacman/db.lck'
which seems to me to be a hint that it happens more than rarely (though not to me yet). I have had the error message, but it quickly cleared as whatever operation was running concluded and released the lock itself - usually I think it was a notification of pending updates that was running automatically… maybe such a thing glitched in your case?