What fantasy does Catherine wish Henry never learn about?

The only event I find that "might" apply to your question is as follows:

Nothing could now be clearer than the absurdity of her recent fancies. To suppose that a manuscript of many generations back could have remained undiscovered in a room such as that, so modern, so habitable!—Or that she should be the first to possess the skill of unlocking a cabinet, the key of which was open to all!

How could she have so imposed on herself? Heaven forbid that Henry Tilney should ever know her folly!