Will this "Someone" even appear?

I am referring to the “Someone is Coming” quality. I have heard people talk about getting a card relating to it once it gets high enough, but I am currently at SiC 8 and nothing has happened yet. Is the card location-specific?

It doesn’t seem to be. I got my card whilst at the Carnival, and other people seem to have been scattered around London. I’d suggest not increasing the Quality any further though, since the excess will just go to waste when you do finally get the card.

It seems to be a fairly uncommon card, but it is well worth the wait! I got 12 echos worth of stuff from one of the options that requires you to have sold your soul previously (and not yet retrieved it).

There is further discussion over here, including information from the developers about the card and its rarity.