I have removed my 3rd row and 2 of the 2nd row with all bolts being torx 55 going into the body, not nuts onto bolts. Furthermore, I have not seen any nut of that particular shape under my vehicle either.
Any chance the cargo area has been used to transport something else which lost the nut.
What mm size is it? There are a lot of large nuts around the control arms and other suspension parts. A mechanic would most likely put removed things into a tray outside though I'd expect, not inside.
That captive washer is interesting. You'd better give the underside a thorough look for any bolts missing a nut.