The Abarth when we were heading to Barmouth? It was coming the other way, away from Barmouth towards Llanelltyd/Dolgellau.
For those that don't know what they look like, it's this (and it was identical to this one except the tinted windows):
The driver was shifting it a little (as you would in that car), and was actually on the wrong side of the white line when I saw it, but I can forgive them as they quickly moved over.
I forgot to mention it when we were stopped.