I feel like all hotel rooms, and hallways in hotels, smell very faintly of cigarette smoke. I don’t find this to be a bad thing; actually, I really like it. Why is this? Did people used to smoke in hotels so much that the smoke just permeated everything and remained even after smoking was forbidden or consigned to separate rooms? Is that what it is? Or is it not smoke, but some other thing - a chemical of some kind? - which creates the nearly-uniform smell that I have encountered in every single hotel I’ve been in, all across the country?
I’ve often wondered this, but never thought to ask. I’ve noticed it too, and only in hotel-type places.
Bumping the thread so someone can enlighten us! 
Yes, *that *smell! It seems to be a combination of stale cigarettes, whatever air fresheners and cleansers the hotel supply companies carry, blended with a piquant touch of mold in the air conditioning system.
The official ban on smoking in hotels, or at least designated rooms, has been around long enough that new hotels have been built, or walls painted, furniture reupholstered, drapes and carpeting replaced - but the smell of stale cigarettes lingers on.
My observation through years of business travel with colleagues, some of whom smoke, is that just because you’re not supposed to smoke in the room doesn’t mean that everyone obeys the rule. I’ve known smokers who requested smoking rooms but were given non-smoking rooms when they checked in because there are fewer designated smoking rooms and none were available. They can’t smoke in the bar or the restaurant, and they’ve got a monkey on their back. So, they go back to their room and light up. I’ve also known smokers who *request *the non-smoking rooms because they are nicer. (Smoking rooms tend to be dingy.) Even if they can open a window while they’re smoking, the stale butts have to be stashed somewhere - and those stink plenty!
All that being said, I do think the cigarette stench was much stronger before the bans went into effect, so it’s doing some good.
Hospitals, which are set up a lot like hotels, have a unique smell, too. I think they substitute alcohol and a hint of urine for the cigarette component.
I also detect a faint old-breakfast-y smell, like bacon, eggs, and toast have been lying around for some time (which, in some cases, they have).
I’ve described the scent of the hallways of Microtel hotels as a peculiar and disquieting blend of off-brand hair conditioner and the water drained off of a can of tuna. Perfume plus emollient plus fish plus metal. Heated up. Just not good.
The nicer the hotel, though, the less offensive the smell, so I do put it down to the quality of the laundry service and the quality of the cleaning materials and cleaning staff.
Most hotels (and all motels) have terrible ventilation.
In your house, most of the doors are open most of the time…and air flows from each room into each other room. In hotels, each room is a totally sealed cube with zero air flow. The hallway is only slightly better: it is a long, narrow tube sealed everywhere except one end which may be open to a stairway or a lobby, but even then has doors blocking the opening. So theoretically air is capable of flowing in or out-- but it usually doesn’t.
Result: stale air, and a funny smell.