Lv2 kara Cheat datta Moto Yuusha Kouho no Mattari Isekai Life - Vol. 5 Ch. 25 - Monster Customer.

[MangaDex]

 

[Aegisx15]

What's in the special museum?

 

[LupusVir]

Is it like some kind of erotica museum?

 

[Nova_Aren]

Oh lol I was wondering why Hiya wasn’t helping.

Yuri route, maybe?

 

[unholy_laugh]

Ok but hold on since what that demon of light and darkess wants to learn even tho it's not specificed it's too clear to not know what that special museum is about

 

[StLOrca]

Look at the decorations around the entrance to the special museum.

 

[Pietje013]

The last panel 😂

 

[Toshkotv]

Futanari doing what she does best

 

[Sola]

Probably a sex museum; a relic of Japan's past.

http://haikyo.org/japanese-sex-museum-hidden-treasures/

http://abandonedkansai.com/2014/12/23/kinugawa-sex-museum-in-nikko-japan/

Still, even if it was underground, in a basement/dungeon like section of the onsen, it's hard to believe those two didn't notice the ruckus going on aboveground.