Deep Dive and the secret ending are two different things.

Deep Dive shows a lot of other things such as a quick scene Kairi in black and white, supposedly two Unknowns talking to each other and Sora flying over the ocean on his back, and is only available in Kingdom Hearts: Final Mix or if you download it.

The secret ending is in the KH that was released outside of Japan and can be accessed by getting the puppies, sealing all of the keyholes and beating the Hades Cup. This is the one that shows the unidentified woman at the end which I'm almost positive is the one that you two are talking about.