...remain in the room while housekeeping is cleaning the room. This isn't the rule but it isn't uncommon. I almost always leave the room anyway when the room attendants show up. Me, I shed body hair in the shower and in bed. I wouldn't want anyone to have to find someone else's hair in either place when I expect to check in to a clean room. It's a well know fact that duvets and bedspreads are washed only infrequently but bed sheets must always be changed at least in between guests.  

I find it pretty odd that these investigators were somehow able to check in to the same room that they were testing. I'm not doubting their methodology but it's my experience that I have no choice of a specific room upon check in except for the room type I've reserved. Even when I've wanted to extend my stay a couple of extra nights and stay in the same room instead of checking out and rechecking in again, it's like a 20 to 30 minute process for the front desk person to be able to make that happen.

And ( an aside observation ), the hotel chains where I tend to stay almost always have a notice that if you toss your towels and washcloths on the floor, they will be replaced. If you hang them on the towel rack it means that you're OK with reusing them. This is to conserve on the water and energy it takes to clean them. In my experience, housekeeping always replaces those items regardless of whether I've hung them or tossed them. SMH...don't understand that one especially within the context of unwashed bed sheets.