Seeing the flooring and steam, this is definitely a Onsen(Hot springs), so being naked is the norm. The only issue is her stockings which Tomo is pointing out anyway, so Koyomi probably just forgot to take them off.
I'm imagining this as Ceci going for her yearly maintenance check, but it's a new mechanic and thus she is shy to show them her chest. Why she's holding a book about boy love is anybody elses guess, maybe the mechanic is Chihaya and maybe Chihaya likes that stuff.
Given the characters involved, the situation, and the completed attack on T-head Admiral by Shirayuki with the grass cutter, this appears to be the direct aftermath of post #3929196
