Taking an educated guess, by the time of the second poster the attraction’s emphasis had moved away from the Bride’s story (as reflected in the less specific new Ghost Host dialogue added shortly after opening) and so the tagline refers to ghostly secrets in a more general sense.
At some point during the development of Phantom Manor, artist Dan Goozée painted this dramatic view of Phantom Manor based on the architectural model by Bob Baranick and various concept art by Fernando Tenedora and Christian Hope, to convey the look and feel of the attraction.
It was not initially meant for the Frontierland attraction posters, which Jeff Burke wanted to look as if they might have come from a Thunder Mesa printing press. He had Maggie Parr create new detailed cross-hatched line art, to be screen printed with graphics and muted colors.
However, somebody in upper management (according to some, Michael Eisner himself) found the final poster too somber to be displayed alongside the other Euro Disneyland posters and insisted on a more colorful and eye-catching design to advertise this major attraction.
With little time to spare, the Imagineers photographed Dan Goozée’s artwork, added bright green graphics and printed lithographs (making it one of only two EDL attraction posters that wasn’t screen printed). One print went under Main St. Station, two others to the ticket booths.
Three prints of the first poster design, which might have already been produced, were nevertheless installed in the park – including any displayed in Frontierland itself as they were designed to fit better into the time period. (One has since been swapped with Main St. Station)
Both posters use similar graphics by Leticia Lelevier, notably Christian Hope’s Rubens-inspired Phantom Manor logotype and additional copy in Caslon Antique… But the first version mysteriously reads “Discover the secret of…” and the second “Discover the secretS of…,” plural.
From Imagineer David Goebel on Twitter
https://twitter.com/One5thParis/status/1354488892251140101