There might've been a Céline store there at one point, back before Phoebe Philo was brought onboard and the brand as a whole was revamped in 2010. There were many more Céline stores around the world back when Michael Kors was designing for the brand, for example, but comparably there are very few official Céline boutiques these days (e.g. New York didn't have one until early 2012, London still doesn't have one). Is the bag you're buying a vintage one from the pre-Phoebe era? If it was, then maybe it was bought at the Castlereagh St store. But if you're buying one of the recent/current designs, it almost definitely wasn't purchased at a Céline boutique on Castlereagh St.