Project Already Open

It occured me a couple of times so far, on opening a project from Dropbox I got this message:

Project Already Open
The project at “…Dropbox/Apps/Scrivener/…” seems already to be in use by another program. If the same project is opened more than once at the same time, data could be lost. It is also possible that this project didn’t finish closing properly the last time it was used. Choose ‘Make a Copy’ to make a copy of the project and work from that, or ‘Continue’ if you are sure that this project is not already open anywhere else.

I have Scrivener on Mac and iPhone, and I the project was closed. I tested Scrivener on Windows at a friend’s place and I am sure I closed everything there too, even logged out of Dropobox.
Is it possible that maybe there is a conflict with Windows and Mac? Or on Windows I should have closed the project in a different way than doing a file save and clicking on the X on the top left-hand side?

This lock is accomplished by using a simple file that is added to the project internals when you open it, and when the project closes, the file is removed. And of course when you open a project, one of the very first things it checks is for that lock file. If it finds the lock file then you get the warning about the project appearing to be open already.

So anything that might cause that lock file to not be removed (or to be restored as might happen with a sync error) when you close the project could cause it to appear open later on. That can happen if the software crashed while it was closing, for example—it never got a chance to properly close the project and so the lock file remains. Or it could be the file is stuck and Dropbox isn’t correctly removing it, or it could be the project wasn’t fully synced to begin with.

The important thing is that this alert is there for your protection against cases where the project really is open. If it’s absolutely not open anywhere else, there is no risk at all in overriding and opening the project with the Continue button.