I was playing Super Mario 3D Land today and noticed I had a Toad House with a familiar Mii... which was odd, as I didn't have a new Streetpass nor had I gone online recently. In any event, I jumped into the Streetpass Mii Plaza and find a new Mii -- Nikki, from Swapnote.
Nikki appeared "six hours ago" by invitation, which was just about when I received a time-locked Swapnote message from an unknown sender.
I assume Nikki arrived with the message -- if you have 'Special Messages' enabled in Swapnote and 'Invite Mii' in the Plaza, I assume you'll be making a new friend too. Does anyone else have Nikki visiting?
Ah, it happened to me too. I didn't check the Plaza yet, but I got a gift from her mii in a toad house in SM3DL.