Every room will have access panels, typically their concealed by decorations or plain panels. On spancrete type buildings you may find holes, like the one in the closet, that had to be cut due to a failure with either plumbing or electrical. Typically the contractor will clean up and cover up their access holes decently, others are just hacks that leave a mess behind. It all depends on how the structure was initially built, if the structure was ever updated or remodeled etc.

Don't get me wrong, the Bates Motel for instance, yeah places like that do exist. One way mirrors, hidden cameras, passage ways in the hidden areas. What I'm getting at is what the Op showed in the video isn't exactly worrisome as you can pretty much tell that there was updates done with the tub surround, and that hole in the floor is for access and not kidnapping someone. How do you figure a full grown adult popping out of there? No way. I'm not saying it cannot happen, but the video here is evidence of access points, however I think I'd think twice before staying in that room with the glass block tub surround, it just looks creepy lol.