I consider my email to be like a personal folder, where I have an expectation of privacy. If I accidentally left my briefcase at your office, I would hope you wouldn't look through it.

If you found the keys to a room full of filing cabinets with other people's papers, it wouldn't be right to go open them all. Similarly, if you find a list of credentials that grant you access to the electronic documents of others, it's not right to use those credentials. Of course 'downloading a car' is a terrible metaphor, but comparing electronic documents to printed documents seems very straightforward.

If we're forcing physical metaphors, I think it's more like finding a stack of 47000 briefcases in an alley somewhere. Unattended, no signs or anything. Opening a bunch of them to just see if people's stolen documents are inside seems reasonable in that case.