You're overthinking it; it'd just be "I found this phone". They don't need to give the backstory. Most businesses will hold onto valuable lost property for a while, on the assumption that a customer dropped something.

I would say that moving the phone from the place I left it would promote it from "lost" to "stolen".

A really key element here is that they offered to help you out by leaving it there for you, so I don't see how "stolen" would come in to it...