Well, I added a lot of tracks in and around the well (The huge bat is using it to exit and enter, after all...) and they found the hidden entrance using a Faerie Fire spell.
BUT... they chose to ignore it, fearing of losing some clues in the manor and, also, fearing the depth of the well (the first climber would be alone for a few moments, before the second could catch up with him)...
So, no need in hiding the PC something... they take all kind of decisions anyway.
My PCs did the opposite, thanks to the Warlock and his Spider Climb-at-will Invocation. They are about to stumble into Aldern with absolutely no clue about who he is and what is going on.