Thank you Jeet!
Yes, no antiques or anything spectacular, it is a tiny studio apartment that is extremely simple and funcitonal.
I was wondering if this was a feature of AirBnB or specific to Japan. The reason is that hte posting itself does not list any security deposit. It also was not listed as a fee, but at the bottom in the corner of hte page, which is why I found it sneaky!
I wanted to post a picture of it here but airBNB requires me to uplaod it elsewhere online first... so I'll bypass that.
I am going to check next time I try to book a place if it is still there.
I'm concerned if it is the case that I might have accidentally agreed to such a condition in prior listings, having been familiar with looking under "security deposit" and varia for all associated fees or risks...