Peepholes — The Window you didn’t know you had
Any hotel will have a peephole through the door, a small tube with a fisheye lens at the outset that allows you to see if there is a pizza delivery person or a murder in a clown suit outside your door.
If you walk up to these peepholes from the outside, and try to look in, it is very hard to see the room — with some fisheye lens correction, you can revert the image to a somewhat original state. Other risks, for example, are if somebody were to loosen it and install a camera into the hole. This isn’t as crazy as it seems.
Lastly, even if you use it the old fashioned way, an observer from outside can see if light is coming through it and becomes obstructed (from you looking through it). This can validate if you’re home, and that you’re on the other side of the door.
I have traveled often for both work and pleasure, and the only hotel chain I’ve seen that installs shutters on their peepholes is Marriott (and Fairfield Inn, owned by Marriott).
Just a shout out to them as a thanks for taking security to the next level.