It could be a complicated legal analysis, like on a bar exam question. As a practical matter, if you snag a used kit, it is saving the local landfill of more garbage. If you snag sealed kits in multitude, the area gets grayer. If you go into wherever the airline stows them, not in public view and not with permission of the crew, then you've probably stolen them.

To whom did the item abandon in your comment? Does it escheat to the airline upon abandonment? To the cleanup vendor? I'd err on "it is petty theft that isn't really actionable" if one takes something that appears abandoned, but may actually have ownership.