There is a bit if a misconception here about Tznius applying only to a place where a Yid sees, and also that Tznius is about being Machshil someone.
The Halachos of "covering up" are not limited to who sees, or if anyone sees. It applies mainly to the type of place; if it is a place that people frequent often enough, then it cannot be done even if there is no one there at the given moment. And it is not about being Machshil someone else or not; it more a statement about the Isha herself, whether she is a Tznuah or a Prutzah. [Source: Kesubos 72 with Meforshim, SHulchan Oruch E"H 115:4.]
[This might make sense logically as well. To give an extreme example: I think most of us would not appear totally Orum in a situation where there is a 1% chance that someone will be watching - unless we're in the Mikvah! Similarly (though of course not exactly the same), in a place where an Isha is supposed to be covered up, she should not be willing to take a chance that no one is there.]
Of course, what is considered to be a place that people frequent often enough is a valid question.
And my point is not to tell you what to do or not, just trying to dispel some misinformation.