of being in an abbey, perhaps?

"An abbey! Yes, it was delightful to be really in an abbey! But she doubted, as she looked round the room, whether anything within her observation would have given her the consciousness. The furniture was in all the profusion and elegance of modern taste."

What strikes me is that Northanger isn't the dark and gloomy place she expected it to be, with "smallest divisions, and the heaviest stone–work, for painted glass, dirt, and cobwebs..." Yet again, we see the discrepancy between what Catherine reads about in Udulpho and reality (and this is something the adaptations get wrong... they make Northanger too much like something out of a gothic novel.) Maybe Catherine is overcompensating for her disappointment, looking for adventure and novelty where there really isn't any to be found. I think this is the one time Catherine's judgement really lets her down. She saw through John Thorpe, she had her misgivings about Isabella, but here her fundamentally good sense has disappeared.

Henry's reaction to her suspicions makes it clear that he thinks she does have a good understanding: "Consult your own understanding, your own sense of the probable, your own observation of what is passing around you." He doesn't need tell her what to think, he only tells her to think about it clearly for herself. She's just let her imagination run away with her and overpower her judgement.