...There is, actually, something, once they're looking closely enough. (At least, if anyone's looking closely enough at the wide variety of erotic - or at least highly suggestive - artworks and decorations.)

Look, art appreciation is a legitimate skill, okay, and Phoe considers herself a very appreciative fan of this genre.

Then in the headboard above the bed, above the attachment points for assorted bonds - where there's an exceptionally gorgeous carving in the bone making up the frame, depicting a woman with her back arched, writhing, her chest pushed out in ecstasy, among numerous reaching arms, skeletal and fleshy alike - if Phoe caresses the woman's chest just so, where there's evidence of her having been caressed many times before -

There's a shadow under the headboard that wasn't there earlier. 

Ooh, secret compartment?

"Think I found something," she calls.

Phae comes over. "What is it?"

Good question. Phoe will investigate.

"It's just under here, I tripped a hidden button or something."

Seems to be a hidden cavity, which contains an unlabeled book! 

"A secret diary or something, maybe."

"What's it say?" 

"No title or author... looks handbound, maybe." Let's see what's inside.

Seems to be a mixture of personal journal - in two different hands - and illustrated guide to sex and kink, commentary and reviews of people the author(s) slept with included. There's some pretty extreme kinks depicted - and, flipping through, a few theorems scribbled in the margins, which at first glance mostly seem intended for enhancing sex - some might be useful outside of sex though, like a theorem scrawled next to a very lovingly detailed drawing of one woman eating another's beating heart, which the tiny cramped note indicates will keep the person it's applied to alive throughout this process. 

Eep! Phoe was not expecting this!

If it's making Phoe react like that... Let her see. 

She passes it over with a blush.

"Fascinating..."

"You think so?"

"What's fascinating?"

"I do think so - both the theorem, and what it's being used for." She passes it over. 

"...Ah. I suppose this explains some things about where the Third's culture came from."

"We're not this bad."

"Not for lack of effort."

"True; if this got back to the Third, there'd be a pretty major revolution in sex."

"So obviously it's our civic duty to test out everything and pass along the best bits?"

"You could make that argument."

"One could also make the argument that this wasn't the sort of hidden secret we were looking for. If one were so inclined."