If he had left the flat completely tidy with no other rubbish then I see your point that alarm bells should have rung however there was no intent from the cleaner because they unboxed it, they didn't act all innocent and handed it back immediately.
I never argued that there was any intent for theft - just that I don't think it would be unreasonable for the cleaner to have at least questioned if it was left behind by accident, given that the OP didn't make it sound as if he'd left the place littered with crap. If that is the case, whilst theft is a harsh description, it is at least somewhat dishonest to find it and not question it, instead just pocketing it and hoping no one asks after it.
I conceded right from the start that if the place was left a dump then you can't expect them to view it any differently from any other junk they found.

