I was on a Princess cruise and was in my cabin on embarkation day, I pulled the curtains all the way back from the balcony and noticed on the floor by the desk there was a passport.
I had just met my steward so I went out into the hall and gave it to him, he said the occupants of the cabin that morning were looking everywhere for it. He said he would bring it to guest services.
Can you imagine the previous occupants blaming the steward for stealing the passport or what if I decided to not turn it in.
It makes zero sense the steward would "steal" your backpack and much more likely someone walking by the cabin took it. Unless, as another poster said, you forgot and left it somewhere else.