I assume they just put it in their suitcase when they leave? At least here they don't check the room until you're gone. People stealing sheets and towels is a classic.
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.
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.
