It's possible you're thinking of "Givenchy", a famous fashion house that Hepburn had a strong connection with. She was famously the face of their fragrance "L'Interdit". This perfume was created specifically for her and is considered one of her signature scents.
It's also possible that you're thinking of "Casque", a perfume by "Jean Patou". Though Hepburn's favorite scent has been confirmed as "L'Interdit", "Casque" was a popular fragrance of the time, and she may have worn it occasionally.
Without more information, it's difficult to be sure which perfume you're thinking of. However, both "L'Interdit" and "Casque" were popular choices during her time.