It might not have been underneath the "floor", afterall, what's underneath the floor of car is the road.
More likely this was found in the spare wheel well, or under a carpet, or if an older car, wedged into a seam underneath a seat, which by very loose definition any cop could say was "hidden under the floor", and which any non-pedant would probably agree.
Have you got kids? And a car? If so, how thoroughly do you clean your car? I've only got the one son, just short of his 12th birthday currently. I traded in a 3 year old Volvo V40 just a couple of months ago. I emptied it every week of junk, I valeted it myself thoroughly every few weeks, and got the local guys to valet it even more thoroughly every few months, but you would not believe the amount of shit one can find beneath almost any loose fitting flap or fairing.
A few years ago I traded in a car (coincidentally, also a Volvo), and though it was almost ready for the scrapper, I valeted it myself, and then also had the local guys valet it for me too, to remove any "evidence". A few weeks later I started getting hounded by the guy who'd bought it at auction and blamed me for buying a car with a faulty clutch. He'd got my number from a torn scrap of MOT invoice that must've been buried so far inside the car I assume he'd had to strip the car down to component parts to find it.
The point is, cars, even when well looked after and regularly cleaned, are black holes for any crap that enters them, especially if you have kids.