Ah yes, the old "mattress in the suitcase" trick.

Oh my god, I feel so stupid. I don't know why I read duvet, not mattress. That's funny though.

My Ikea mattress came rolled up very small. Not quite suitcase-sized, but if you find a way to get close to such a size using simple tools (something you can fit in a backpack or carry-on), I can see this happening. One wouldn't need a human-size suitcase to pull that off.

Note that this is based on mental estimations of a ten year old memory. I may be off about how small it was, but I think the estimation should be accurate in essence.

Some mattresses can be rolled. At least IKEA sells them that way.

https://www.youtube.com/watch?v=8DQlKDSvK7M

Hotels should be suspicious of guests with extremely enormous luggage.

Sheets and towels can fit in a suitcase. I don't know of anyone who has a suitcase large enough for a mattress.

I also find the comment guests "guests will most likely attempt to take the mattress at night when the reception desk isn’t open" odd, since most four/five star hotels usually have a front desk that is staffed 24 hours.

Actually I was wondering that too ... but I've also gotten several mattresses in the mail, and while the package isn't precisely "suitcase" sized, it's not that much more than "extra-large sports duffel" sized.

I wonder if some of these are outside of busy metros. Lots of non-metro 5 star hotels that aren't open 24/7

If they aren‘t open 24/7, they are at most 3 star superior hotels, no matter how luxurious they are (at least around here).