Originally Posted by Martian

The rational explanations are boring, because they attribute everything to the mundane. They're also always right. Having dabbled in mentalism myself, I've seen that 'how the fuck did you do/know that?; look, and it can be quite powerful (as well as a great way to pick up, but that's beside the point). The technical term for what is most probably going on here is what's referred to as a hot reading. This is in contrast to the much more common cold reading; hot readings are harder to do, because they require some foreknowledge on the mark. All the same, it's not difficult to imagine that this gent overheard this expectant mother speaking with one of her other daughters out on the street and followed them into the store specifically to have a bit of a game and get that reaction.

That said, believe whatever makes you happiest. You're never going to get a complete answer short of tracking the old guy down and that seems a bit extreme.